Do you think it is fine for people in their nineties to be driving?

I love really old people with snow white hair. I was in the grocery line and in front of me was a smiling lady that I know was past her eighties. She was alone, chatting with the cashier, joking about nursing homes (saying she may be ready or something) and swiped her card. I was behind her and noticed her getting into her car and driving off. Do you think that she is more a danger to herself and others than younger drivers? Or that people should be made to take a driver’s test when they hit ninety and each six months thereafter? I frankly had an uneasy feeling.