There's a theory making the rounds (still a theory, although there have been papers written on it) that all neurodegenerative diseases are really variations of "Type III diabetes." This fits in neatly with the mitochondrial explanation that has been mentioned on other threads, as the consequences of inflammation produced by insulin resistance and too much circulating glucose is ultimately cellular damage due to reactive oxygen species (ROS).

My WAG is that whether you get PD, Alzheimer's, multiple sclerosis, ALS, or Huntington's will depend on your personal genetic makeup. If you get PD, then how early your nigral cells start to die also depends on your individual sensitivities - just as not all people who drink eight Cokes a day go on to develop Type II diabetes, and not all people who smoke get lung cancer.