My sis-in-law is in her 50s and has had Parkinson's for maybe 15 years now. It is so hard watching the decline over time. She can still walk, drive etc but has times that she just freezes up and can't walk. The dr does keep adjusting meds but nothing seems to help for long anymore. In her case her uncle also had Parkinson's and developed Alzheimer's Disease. Though that isn't talked about (to me at least) I know that has to be a fear as well.
My fil is in his 80s and has Alzheimer's Disease which presented first then developed Parkinson's Symptoms. Although he is older it is still hard seeing the changes. In terms of Parkinson's I know he has some trouble walking especially on even surfaces and he absolutely loves walking. He lives in Manhattan so walking is so common and even though he has no short term memory he wants to walk all the time. I hate both Parkinson's and Alzheimer's.