Some people are healthy in their old age and some are all done in their 50's and 60's. A local retirement home is full of people in their 50's and 60's that have walkers, self propelled scooters and wheel chairs. They're done. And the theme is pretty consistent, smokers, drinkers, no exercise to speak of, fast food diets and fat. You hear them say things like, I might as well be dead if I have to give up smoking, drinking and my favorite food. And so, they get their wish.
Very few others are lucky enough to have good genetics and get away with unhealthy life styles but those are the few. And of course don't forget the few that live a healthy, active life style and are good to go long after their peers have faded away.
These people that are unhealthy and all done should not be driving. It's just too bad that there's no fair way to figure this out other than the obvious ones that should absolutely be shut down.