Well, the elderly shouldn't be limited as to means of transportation based on their age. If they are still able to follow the rules of the road and can still properly handle a car, then it really should not be an issue. Sure, they're old, and they might drive a little bit slower than us speed demons, but hey, that's why most roads have more than one lane. Don't feel comfortable around old drivers? Go ahead and pass them.

But hey, I think that most people, whether they are old or young, have the ability to tell whether or not they are able to handle driving a car.
I bowl with this woman - she's well into her eighties, and she still has quite an active social life. I drive her to bowling on Friday nights since she lives around the corner from us. She is able to run her own errands and even does volunteer work. Her husband is 10 years deceased and she lives alone...yet she still has the strength to do her own shopping, housework, etc. She even puts up a Christmas tree yearly.

AND she drives, too...but she does feel uncomfortable driving at night so this is why I do the driving on Fridays.
Point is, even though she's old, she knows her limits. I wish more people would do that, ya know? I dunno...there probably won't ever be a cut off age, but I think as people get older and older, their desire to drive will eventually cease and then they'll stick to walking or taking a bus or calling a younger grandchild or relative to come and pick them up.
Just my thoughts anyhow.