One clear risk factor for Parkinson's disease is age. The average age of the onset of the disease is 60 years, and the incidence rises significantly with increasing age. However, about 5-10 percent of people with Parkinson's disease have an "early-onset" of the disease, which begins before the age of 50. Early-onset forms of the disease are often, though not always, inherited, and some have been linked to specific gene mutations. People with one or more close relatives who have Parkinson's disease have an increased risk of developing the disease themselves, but the total risk is still just 2-5 percent unless the family has a known gene mutation for the disease. An estimated 15-25 percent of people with Parkinson's disease have a known relative with the disease.
In rare cases, parkinsonian symptoms may appear in people before the age of 20. This condition is called juvenile parkinsonism.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.