My dad had macular degeneration and continued to drive for a few years. He had always been an amazingly good driver, and even with his poor vision I felt very comfortable riding with him. If I hadn't known for a fact that his vision was failing I wouldn't have been able to tell by the way he drove.
Not so my mom, who always made me nervous, even when she was much younger. If I had a choice I always rode with dad. He gave up his keys voluntarily and sold his car, and let mom ferry him around, which I took for certain blindness, because he couldn't see the near misses, or a death wish.