[SteveFromNY]

I agree this is a tough one.
Some people should probably hang up their car keys, but age affects people so differently. Google "marathon martin 101" and read about Buster Martin, a Brit who is training for the Boston Marathon. I realize driving skills are much different than running skill, but I only mean to point out that age shouldn't be the only factor. I have folks in my family (I won't say who in case they see this!) who are relatively young who drive HORRIBLY! They should hang up their car keys too!! They are just plain bad drivers!! What to say to them?
I like the idea about the doctor helping. That's someone who might get enough respect to have an effect. But make sure the advice is being given to a person who listens to their doctor - not everyone does!