Young-onset Parkinson's disease: a clinical review. Young-onset Parkinson's disease (YOPD) is arbitrarily defined as that which produces initial symptoms between the ages of 21 and 39, inclusive. The special problems and concerns of the patient with YOPD present as much of a challenge and opportunity for the clinician as the disease itself does for the researcher. In contrast to juvenile parkinsonism, which is a heterogeneous group of clinicopathologic entities presenting (also arbitrarily) before age 21, YOPD appears to be the same nosologic entity as older-onset PD. It comprises approximately 5% of referral populations in Western countries and about 10% in Japan. Its annual incidence relative to the population at risk is about 1/10 that of PD at age sixty. YOPD tends to have more gradual progression of parkinsonian signs and symptoms, earlier appearance of levodopa-related dyskinesias and levodopa-dose-related motor fluctuations, and frequent presence of dystonia as an early or presenting sign. Studies conflict with regard to the suspected greater familial frequency and lesser frequency of dementia than in older-onset PD. 