It's tough, my 72 year old mom is still one of the best drivers I know. She still drives how I think of as "aggressively well" (for example she aggressively accelerates into traffic when needed and is always aware of what's going on well in front and behind her). My grandma, on the other hand, was scary as hell. Turns out she was slipping into dementia well before it was obvious to anyone around her. She's in a home now but at the time they would have had to take her car to keep her from driving.