I dont think we can put a restriction on in based on age. Some people can be 100 and still be fine - have good brain functions and vision, while others can be 70 and not safe to drive.
It really depends. Vision is the main one - if you cant see properly then you cant drive safely. I would put in complusory vision checks for everyone over a certain age (maybe 65). In terms of brain function this is harder to measure, but maybe there needs to be some tests being done...
When your eyes are too old to see clearly and your limbs are too old to manage the controls
I would say when you don't have the strength to drive or when you are very weak with loss of strength in addition to weak eye sight, then you are too old to drive.
Thanks for reading and I hope this helps