Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder after Alzheimer's disease and represents a major global public health challenge due to its increasing incidence and associated disability. The disease is characterized by the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, resulting in dopamine depletion within the nigrostriatal pathway and the accumulation of intracellular α-synuclein aggregates known as Lewy bodies. These neuropathological changes give rise to a combination of motor manifestations, including bradykinesia, resting tremor, muscular rigidity, and postural instability, together with a broad spectrum of non-motor symptoms such as cognitive impairment, depression, sleep disturbances, autonomic dysfunction, anosmia, and psychiatric complications. The growing burden of PD has become a major concern because of increased life expectancy and the ageing global population, making it one of the fastest-growing neurological disorders worldwide[1-3]. 

Although Parkinson's disease has traditionally been regarded as a disorder of older adults, substantial clinical heterogeneity exists based on the age at disease onset. Approximately 5–10% of patients develop symptoms before the age of 50 years and are classified as having Early-Onset Parkinson's Disease (EOPD), whereas the majority of cases present after 60 years of age and are categorized as Late-Onset Parkinson's Disease (LOPD). Increasing evidence indicates that these two clinical phenotypes differ considerably with respect to epidemiology, genetic susceptibility, molecular pathogenesis, clinical presentation, therapeutic response, and long-term prognosis. Consequently, age at onset has emerged as an important determinant of disease phenotype and clinical management [4-7].

The biological mechanisms underlying EOPD and LOPD are also distinct. Early-onset disease is frequently associated with inherited mutations in genes involved in mitochondrial function and protein homeostasis, including PRKN (Parkin), PINK1, DJ-1 (PARK7), and LRRK2. These genetic abnormalities contribute to impaired mitochondrial quality control, oxidative stress, defective autophagy, and neuronal vulnerability at a younger age. In contrast, LOPD is predominantly influenced by age-related neurodegenerative processes, including mitochondrial dysfunction, chronic neuroinflammation, impaired proteostasis, oxidative damage, and cumulative environmental exposures. These mechanisms collectively accelerate neuronal degeneration and contribute to the more aggressive disease course observed in older individuals [8-12].

Clinically, patients with EOPD often exhibit slower disease progression, better preservation of cognitive function, and a more favourable response to dopaminergic therapy during the early stages of disease. However, because of prolonged exposure to levodopa, they are more susceptible to developing levodopa-induced dyskinesia and motor fluctuations over time. Conversely, patients with LOPD generally experience more rapid motor deterioration, postural instability, gait impairment, cognitive decline, autonomic dysfunction, hallucinations, and increased functional disability. These age-related differences have important implications for individualized therapeutic decision-making, long-term follow-up, and healthcare planning [13-17].

The socioeconomic impact of Parkinson's disease also varies according to the age of onset. Individuals diagnosed with EOPD frequently develop symptoms during their economically productive years, resulting in substantial indirect costs associated with loss of employment, reduced productivity, caregiver burden, and impaired quality of life. In contrast, LOPD is associated with higher direct healthcare expenditures due to hospitalization, management of multiple comorbidities, institutional care, and increasing dependency during advanced stages of the disease. Recognition of these contrasting economic and psychosocial consequences is essential for developing effective healthcare policies and patient-centred management strategies [18–20]. 

Despite the increasing recognition of age-related heterogeneity in Parkinson's disease, many published studies continue to evaluate PD as a single clinical entity, thereby limiting the understanding of important biological and therapeutic differences between EOPD and LOPD. Furthermore, inconsistencies in diagnostic criteria, age-at-onset definitions, and outcome measures have hindered direct comparison among available studies. A systematic synthesis of the current evidence is therefore necessary to clarify the differential epidemiology, pathophysiological mechanisms, clinical manifestations, treatment outcomes, and socioeconomic burden associated with these two phenotypes [21-23]. 

Therefore, the present systematic comparative review aims to critically evaluate and integrate the available evidence regarding the epidemiology, genetic architecture, molecular pathophysiology, clinical characteristics, therapeutic strategies, and long-term outcomes of Early-Onset and Late-Onset Parkinson's Disease. By identifying similarities, differences, and current knowledge gaps, this review seeks to provide a comprehensive scientific framework that supports precision medicine approaches, facilitates individualized patient care, and highlights priorities for future clinical and translational research [24-26].

Protocol and Reporting Standards:

This comprehensive comparative analysis was conducted following the guiding principles of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement, ensuring methodological transparency and rigor of the study. This study aimed to systematically collate, evaluate, and synthesize data from published studies comparing EOPD and LOPD cohorts 29.

Screening:

Note: Independent screening was conducted based on the PICOS framework. 

Eligibility:

Note: Assessed against full inclusion/exclusion criteria. 

Full-text reports excluded (with reasons): (n = 88)

Inclusion:

The literature search and inclusion criteria were defined a priori based on the following Population, Intervention/Exposure, Comparator, Outcome, and Study Design (PICOS) framework:

Population (P):  Studies enrolling patients diagnosed with Parkinson’s disease (PD). 

Exposure/Comparator (I/C):  Studies explicitly comparing cohorts based on Age at Onset (AAO). The defined threshold for   early onset  Parkinson’sdisease  (EOPD) was AAO after 21 years but before 50 years , and the  Late-Onset Parkinson’s  disease  (LOPD) comparator cohort included patients with an AAO of 50 years or older.    

Outcomes (O):  Key outcomes of interest included but were not limited to: 

Epidemiological Trends:  Incidence, prevalence, mortality, and Disability-Adjusted Life Years (DALYs) stratified by age and sex.     

Etiology and Genetics:  Prevalence of monogenic determinants (e.g., PRKN, PINK1, LRRK2)  and non-genetic risk factors (e.g., head injury, smoking).    

Clinical Phenotype:  Comparison of initial motor symptoms (tremor vs. rigidity), non-motor symptom profiles (ICDs, psychosis, cognitive status), and disease progression velocity.     

Therapeutic Response:  Rates of Levodopa-Induced Dyskinesia (LID) and motor fluctuations and candidacy/outcomes for Deep Brain Stimulation (DBS).    

Socioeconomic Impact:  Quality of Life (QoL) metrics (PDQ-39) and occupational/economic burden.     

Study Design (S):    This included comparative cohort studies, large-scale epidemiological analyses, systematic reviews, and meta-analyses. Single-case reports and studies without explicit comparison groupswere excluded. 

Key Alignment Details:

Standardized AAO Cutoffs (Exclusion Reason 1): Your exclusion of 34 reports at the eligibility stage based on standardized Age-at-Onset (AAO) cutoffs perfectly addresses the potential bias from inconsistent data noted in your Methodology (Section 2.4).

Data Heterogeneity (Section 2.5): The inclusion box explicitly breaks down the final 38 studies, showing how many contributed to the quantitative data in your comparative tables (n=14) versus how many were synthesized via narrative review only (n=24) due to heterogeneity. This directly supports your stated synthesis plan ([Fig. 1]).

An exhaustive search was conducted across major biomedical literature databases, including PubMed/MEDLINE, Web of Science, and Google Scholar. The search strategy employed combinations of Medical Subject Headings (MeSH) terms and keywords, such as: ("Parkinson's Disease" OR "Parkinson Disease") AND ("Early-Onset" OR "Young-Onset" OR "EOPD" OR "YOPD") AND ("Late-Onset" OR "Old-Onset" OR "LOPD"). The search was not restricted to publication date to ensure a comprehensive overview of historical and recent comparative literature. Epidemiological data were obtained from robust population-based cohort reports and global burden studies.   

Titles and abstracts were independently screened for relevance based on the PICOS criteria. Full-text articles were retrieved and assessed for final inclusion. Data extraction was performed using a standardized instrument to capture key variables (e.g., AAO cutoffs, sample size, and outcome measurements). The methodological quality was assessed based on the study design. Particular attention was paid to potential sources of bias, such as selection bias in hospital-based records versus population-based registries and interobserver variability in clinical assessments.   

Owing to the heterogeneity of outcome measures, study designs, and AAO cutoffs observed across the literature, the primary approach to data synthesis was a detailed narrative review. Quantitative data, where available and consistently reported (e.g., prevalence rates, genetic frequencies, motor complication rates), were synthesized to quantify the magnitude of differences and presented in comparative tables to facilitate interpretation. The synthesis focused on identifying consistently reported phenotypic and etiological distinctions to establish the core comparative profiles of EOPD and LOPD.   Only studies with acceptable methodological quality were included in the final evidence synthesis. 

The studies included in this systematic review demonstrate that age at disease onset is a major determinant of the epidemiological and clinical heterogeneity of Parkinson's disease (PD). Early-Onset Parkinson's Disease (EOPD), generally defined as disease onset before 50 years of age, represents a relatively small proportion of PD cases, whereas Late-Onset Parkinson's Disease (LOPD) accounts for the majority of diagnoses. Despite its lower prevalence, EOPD is associated with a longer disease duration and distinctive clinical characteristics that differentiate it from LOPD. 

The reviewed literature consistently indicates that genetic susceptibility contributes more significantly to EOPD, while LOPD is primarily influenced by ageing-related neurodegenerative processes and cumulative environmental factors. Consequently, patients with EOPD often present during their economically productive years, whereas LOPD predominantly affects older adults and is associated with a greater burden of age-related comorbidities. 

Although EOPD patients generally experience a slower rate of disease progression, their longer survival results in prolonged exposure to dopaminergic therapy and an increased lifetime risk of treatment-related motor complications. In contrast, patients with LOPD typically exhibit faster clinical deterioration, greater functional impairment, and higher healthcare utilization because of advanced age and accompanying systemic disorders ([Fig. 2]). 

The reviewed evidence highlights clear molecular differences between EOPD and LOPD. EOPD demonstrates a stronger hereditary component and is frequently associated with pathogenic variants in genes involved in mitochondrial maintenance, oxidative stress regulation, and intracellular protein degradation. Among these, PRKN, PINK1, DJ-1, and LRRK2 are the most consistently implicated susceptibility genes. 

In contrast, LOPD appears to result predominantly from multifactorial mechanisms involving ageing, mitochondrial dysfunction, impaired proteostasis, chronic neuroinflammation, oxidative stress, and prolonged environmental exposure. These age-related biological changes accelerate dopaminergic neuronal degeneration and contribute to the more rapid disease progression observed in older patients. 

Overall, the findings indicate that although both phenotypes share common neuropathological features, their underlying molecular mechanisms differ considerably, emphasizing the need for age-specific diagnostic biomarkers and targeted therapeutic strategies.

Comparison of the included studies revealed marked differences in both motor and non-motor manifestations between EOPD and LOPD. Patients with EOPD generally exhibit slower progression of motor symptoms and retain functional independence for a longer period. They also demonstrate better cognitive preservation during the early stages of disease and respond more favourably to dopaminergic therapy. However, because of prolonged treatment duration, these patients experience a higher incidence of levodopa-induced dyskinesia and motor fluctuations. 

Conversely, LOPD is characterized by more rapid motor deterioration, postural instability, gait impairment, cognitive decline, psychosis, autonomic dysfunction, and increased disability. Older patients frequently require earlier caregiver support and experience greater reductions in quality of life because of progressive functional dependence. 

These findings demonstrate that age at onset significantly influences both the clinical phenotype and long-term disease trajectory of Parkinson's disease.

The reviewed studies indicate that levodopa remains the cornerstone of symptomatic treatment for both EOPD and LOPD. Nevertheless, therapeutic response and long-term treatment outcomes differ according to age at onset. Patients with EOPD generally show excellent initial responsiveness to levodopa but are more likely to develop dyskinesia and motor fluctuations because of prolonged cumulative exposure. Consequently, dopamine agonists and other levodopa-sparing strategies are often considered during the early stages of treatment in younger patients. 

In contrast, patients with LOPD often require individualized dose adjustment because of increased frailty, multiple comorbidities, cognitive impairment, and greater susceptibility to adverse drug reactions. Therapeutic decisions in older adults therefore focus on optimizing symptom control while minimizing treatment-related complications and maintaining functional independence. 

Among advanced treatment options, Deep Brain Stimulation (DBS) remains the most extensively studied surgical intervention. The reviewed evidence indicates that carefully selected patients with EOPD derive greater long-term benefit from DBS owing to their younger age, preserved cognitive function, and lower burden of systemic comorbidities. DBS effectively improves motor symptoms, reduces medication requirements, and enhances quality of life in appropriate candidates. 

Although selected patients with LOPD may also benefit from DBS, treatment decisions require careful evaluation of cognitive status, medical comorbidities, and surgical risk. Overall, patient selection remains the principal determinant of favourable clinical outcomes following advanced therapeutic interventions. 

The included studies demonstrate that the socioeconomic consequences of Parkinson's disease differ substantially according to age at onset. EOPD predominantly affects individuals during their most productive working years, resulting in considerable indirect costs related to reduced employment, productivity loss, premature retirement, caregiver burden, and diminished quality of life. 

In contrast, the economic burden associated with LOPD is driven mainly by direct healthcare expenditure, including hospitalization, long-term medical care, rehabilitation, institutional support, and management of multiple age-related comorbidities. Increased dependency and progressive disability further contribute to healthcare utilization among older patients. 

Collectively, these findings highlight that EOPD and LOPD represent clinically and economically distinct phenotypes of Parkinson's disease. Recognition of these differences is essential for individualized treatment planning, healthcare resource allocation, and the development of age-specific management strategies [Table. 2].

This systematic comparative review synthesizes current evidence demonstrating that EOPD and LOPD represent distinct pathophysiological and clinical phenotypes. EOPD is characterized by a strong monogenic basis, isolated nigrostriatal degeneration, high neuroplastic compensation, volatile levodopa pharmacodynamics, and substantial indirect economic burdens. LOPD is defined by multifactorial, age-related degeneration affecting widespread cortical, limbic, and cholinergic circuits, leading to early cognitive decline, dysautonomia, and high direct healthcare costs.

The present systematic comparative review highlights that Early-Onset Parkinson's Disease (EOPD) and Late-Onset Parkinson's Disease (LOPD) represent distinct clinical phenotypes with significant differences in epidemiology, genetic susceptibility, pathophysiological mechanisms, disease progression, therapeutic response, and 

socioeconomic burden. The findings consistently demonstrate that age at disease onset is an important determinant of disease heterogeneity and should be considered when planning diagnostic evaluation, therapeutic interventions, and long-term patient management[4-7]. 

One of the most consistent observations across the reviewed studies is the stronger genetic contribution to EOPD compared with LOPD. Pathogenic variants in PRKN, PINK1, DJ-1, and LRRK2 were frequently associated with younger patients, whereas late-onset disease appeared to be predominantly influenced by ageing-related biological changes, including mitochondrial dysfunction, oxidative stress, impaired protein degradation, and chronic neuroinflammation[8-11]. These findings support the concept that Parkinson's disease is not a single homogeneous disorder but comprises biologically distinct subtypes with different underlying molecular mechanisms. Consequently, genetic evaluation may be particularly valuable in younger patients presenting with a family history or atypical clinical features[9, 11]. 

The comparative analysis further demonstrates important differences in clinical presentation and disease progression between the two phenotypes. Patients with EOPD generally experience slower progression of motor disability, prolonged preservation of cognitive function, and better responsiveness to dopaminergic therapy during the early stages of disease. However, prolonged exposure to levodopa substantially increases the lifetime risk of motor fluctuations and levodopa-induced dyskinesia. Conversely, patients with LOPD exhibit more rapid deterioration of motor function, earlier postural instability, greater cognitive impairment, autonomic dysfunction, and increased functional dependence, which collectively contribute to poorer clinical outcomes and reduced quality of life[12-16]. 

Therapeutic management should therefore be individualized according to age at disease onset. Although levodopa remains the most effective symptomatic treatment for Parkinson's disease, younger patients often benefit from levodopa-sparing strategies during the early phase of treatment to delay the development of motor complications. In contrast, treatment of LOPD requires careful dose optimization because older patients are more susceptible to adverse drug reactions, cognitive impairment, orthostatic hypotension, and drug-related hallucinations. These observations reinforce the importance of balancing symptomatic control with long-term treatment safety when selecting pharmacological interventions[15-18]. 

The reviewed evidence also supports the role of Deep Brain Stimulation (DBS) as an effective advanced therapeutic option, particularly in carefully selected patients with EOPD. Younger patients generally achieve greater improvement in motor symptoms, reduced medication requirements, and better long-term functional outcomes following DBS because of preserved cognitive function and fewer systemic comorbidities. Although selected patients with LOPD may also benefit from surgical intervention, comprehensive preoperative evaluation remains essential to minimize perioperative risks and optimize postoperative outcomes[19-21]. 

Beyond clinical outcomes, this review highlights considerable differences in the socioeconomic burden associated with age at disease onset. EOPD frequently affects individuals during their most productive years, resulting in substantial indirect costs due to reduced employment, premature retirement, caregiver burden, and loss of productivity. In contrast, LOPD is associated primarily with increased direct medical expenditure related to hospitalization, rehabilitation, long-term institutional care, and management of multiple comorbidities. Recognition of these distinct economic challenges is essential for healthcare planning and resource allocation[22–24]. 

The present review has several strengths. It provides a comprehensive comparison of EOPD and LOPD by integrating epidemiological, genetic, molecular, clinical, therapeutic, and socioeconomic evidence within a single systematic framework. This integrated approach facilitates a broader understanding of age-related disease heterogeneity and supports the growing emphasis on personalized medicine in Parkinson's disease management[4, 6, 20]. 

Nevertheless, several limitations should be acknowledged. The included studies exhibited considerable heterogeneity in study design, sample size, age-at-onset definitions, follow-up duration, and reported clinical outcomes, limiting direct comparison across investigations. Furthermore, variations in diagnostic criteria and methodological approaches prevented quantitative data synthesis. Therefore, the findings of this review should be interpreted within the context of the available evidence, and future studies employing standardized definitions and multicentre prospective designs are warranted to strengthen the evidence base[25-27]. 

Future research should focus on identifying reliable biomarkers for early diagnosis, improving genotype-phenotype correlation, and developing precision medicine approaches tailored to different age-at-onset groups. Advances in molecular genetics, neuroimaging, artificial intelligence, and disease-modifying therapies may facilitate earlier diagnosis and more individualized therapeutic strategies. Longitudinal studies evaluating long-term clinical outcomes and cost-effectiveness of personalized treatment approaches are also required to improve patient care and optimize healthcare resource utilization[28-30]. 

Overall, the evidence synthesized in this systematic review demonstrates that EOPD and LOPD differ substantially in their biological basis, clinical phenotype, therapeutic response, and socioeconomic consequences. Recognition of these differences is essential for implementing age-specific diagnostic strategies, optimizing individualized treatment plans, and guiding future translational research aimed at improving the quality of life and long-term outcomes of patients with Parkinson's disease[4-30].

Recognizing these distinct phenotypes has clear clinical implications for daily patient care: 

In EOPD: First-line pharmacological strategies should focus on delaying motor complications by employing dopamine agonists, MAO-B inhibitors, or COMT inhibitors where appropriate, paired with regular screening for ICDs (compulsive gambling, shopping, and hypersexuality). When motor fluctuations become refractory, early evaluation for DBS or continuous infusion systems should be prioritized to preserve functional and vocational capacity. 

In LOPD: Management should focus on conservative levodopa titration to maintain mobility while avoiding drug-induced hallucinations, orthostatic hypotension, and confusion. Physical therapy for balance and fall prevention, along with routine cognitive monitoring, should take precedence over aggressive motor escalation.

Several limitations of the available literature should be noted. Considerable methodological heterogeneity exists across published studies regarding AAO cutoffs (ranging from 40 to 55 years), clinical rating scales, and follow-up durations, which limited quantitative meta-analysis across all outcomes. Furthermore, tertiary movement disorder centers introduce potential referral and publication biases by over-representing severe or genetically complex EOPD cases relative to population-based registries. Standardized AAO definitions and larger multi-center prospective registries are needed to resolve these disparities.

EOPD and LOPD represent clinically and biologically distinct phenotypes of Parkinson's disease. Age at onset serves as a key determinant of underlying genetic architecture, disease trajectory, and therapeutic response. While EOPD requires therapeutic strategies focused on 

managing motor complications, preserving vocational capacity, and offering targeted psychosocial support, LOPD necessitates careful pharmacological titration to mitigate cognitive decline, psychosis, and fall risks. Advancing precision medicine in PD will require standardized diagnostic cutoffs, validated biomarker discovery, and clinical trials stratified by age at onset. 

Funding: The authors received no specific financial support for this work.

Conflict of Interest: The authors declare that there are no conflicts of interest.

Ethical Approval: Not applicable. This systematic review was conducted using previously published literature and did not involve human participants or experimental animals.