Do only old people get parkinson's disease?

No: Though patkinson's more commonly occurs in the elderly, it can sometimes start in people much younger (eg michael j fox developed pd by his very early 30's).

No-age varies widely: 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 inheritable & mild. Either way, all must be treated promptly by aneurologist with the best modern approach meds to maximize daily function, health & qol!