I know in CA when I had to renew my drivers license a few years ago I had to take a vision test again.

As for you're mother if she is squinting to read a book or magazine etc sounds like she is nearsighted which does not necessarily mean she will have problems driving. Alot of areas to offer alternate transportation for the elderly. I know before my grandmother passed away, she used one of these services (sorry can't remember the name, has been quite a few years). You have to make appointments with them normaly a few days ahead of time but it is only a few dollars to go virtualy anywhere.