How old before too old to drive a boat?

Never. Safe boating depends entirely on your ability to handle the boat mentally and physically. I know a man who still circumnavigates his trimaran at 80 years old. There is no age limit.
Same as a car. If the physical capacity limits the ability to maneuver the boat or the vision/hearing limit the ability to see and hear hazards, then that person is too old to conduct a boat.