Yeah about that... "Parkinson's disease is caused by a loss of nerve cells in the part of the brain called the substantia nigra.". Also - scientists believe its a mix factor of genetics, toxins, and environmental factors that cause it.
Let's take Michael J Fox. Didn't happen until later in life when it began to sink in for him. Then it became so prominent it's hard for him to work a job. But at some point in his adult life - it sank in.
Now for Joe Biden. Biden had brain problems many years back, to where they surgically removed about an ice cream scoop worth of brain, from Joe Biden's head. Meaning somewhere between 2020 and 2024, it would have been about the right timing for Parkinson's to develop after his Brain Surgery where they removed some brain.
His environment of his brain changed in that surgery and there's possibly a genetic predisposition.