Young or old. Go to http://www.Pdf.Org/ the mean age of onset is about 65 years. Cases occurring in ages 21 to 40 are considered young-onset pd; those younger than 21 have juvenile parkinsonism.  from ages 0 to 29, the average annual incidence rate of parkinsonism is 0.8/100, 000 person-years, and rates increase incrementally to 304.8/100, 000 person-years in those aged 80 to 99 years. .
30's or 60's. If you have the aggressive familial type (michael fox), it may start 30-40. More typically, 60s-70's. Look for non-intention tremors (tremor when that area, usually thumb, is not being used) on one side of body; bradykinesis: slowing of all movements, less ability to show facial expressions, .
Further Detail. Average age of onset is about 58-60 years old. However, true pd can be seen as early as late 20's to as late as 80-90's. Typically, the younger the age of onset, the more genetically inheritable & severe. The later the onset the less genetically i heritable & mild. Either way, all must be treated promptly by aneurologist with the best modern approach meds to maximize daily function, health & qol!