Unfortunately, people don't want to do an honest evaluation of their abilities and capabilities. This, in itself, is probably the most obvious clue that you shouldn't continue. As has been said, it's not a matter of years, but physical and mental abilities. Some clues might be more accidents, more near misses, losing or leaving more stuff at the CG, and more navigating mistakes. Also greater and greater difficulty in parking the RV. I would hope I could recognize these things in myself, but I also have a partner of over 53 years that will help. It's too bad there are so many out there that are well past their cutoff point. I do my best to stay as far from them as possible.
By the way, this is not unique to RVing. I ran the Delaware 55 ALIVE program for a number of years and just getting seniors to hang up their car keys due to failing abilities is a far greater problem overall. No easy solutions.