How old is too old when it comes to driving? When is it time for seniors to give up their driving privileges?

My 83 year-old grandfather was driving nearly until he died. Had it not been for a sudden head injury caused by a fall he took at a convenient store that led to a stroke, he might still be driving today. Pop-Pop as I affectionately called him was a retired Lt. Colonel from the U.S. Army. He was wonderful with people, loved and was loved by everybody.