The incidence of Parkinson’s disease rapidly increases over the age of 60 years–with only 4% of cases occurring under the age of 50.
Van Den Eeden S, Tanner C, Bernstein A, Fross R, et al. Incidence of Parkinson’s Disease: Variation by age, gender, and race/ethnicity. Am J Epidemiol. 2003; 157(11): 1015-22. http://aje.oxfordjournals.org/cgi/content/full/157/11/1015