As long as they stay off the road, or at least stay in the right lane, I have no problem with anything old people do....
But for the seriousness of this topic:
It's tough to see a loved one degrade with age. It's tougher for them. One thing I'd recomend is to have a chat with them, but be very careful. Elders don't like to hear advice from people they raised. It's tough for the parent to accept help from their child, no matter how old they get and no matter what experience their child has.
I had some luck with my grandpa, but my dad could just never get through to him.