W
Answering Marys core question is in Marys answer?
She was still instructing at the ripe age of 77 and gave up when her health deteriorated due to a stroke.
Fair dues to Mary she still takes an active interest by imparting her knowledge on the ground and helping around the club as well as flying with an instructor.
Most people know when to quit! Either they no longer want to fly or they know their health is not as good as it should be which usually goes back to not wanting to fly.
There are probably a small minority who refuse to acknowledge their lack of health or declining abilities but usually others do notice.
they still have to pass a medical and maybe over 70 should have more tests or slightly different flight tests as maybe should car drivers.
with commercial pilots any decline in performance would easily be picked up under pressure and mental loading in a simulator
Pace