People age at very, very, very different rates....
When I became a helicopter instructor in my early 50s, people told me it was a young person's job, especially on such demanding, responsive machines as the R22 helicopter. I had absolutely no idea what they meant. My stamina, reaction speed, memory etc seemed to be the same as they had been in my 20s. I loved the long hours airborne on good weather days, and would happily have flown 7 days a week.
By a few years later, I still loved it, could still do it effectively, but used to get home utterly exhausted. I looked forward to days off, and the day I woke up on a bad weather day and thought, "Thank God I don't have to fly today" was the day I realised something had changed.
By the time a combination of the recession, flying school closing, and some minor medical issues meant I had to take a break from instructing, I was relieved. I was as sharp as ever most of the time, but.... Something wasn't the same. What was it? Was I missing little things, taking longer to react? I wasn't sure, but I didn't feel 100% safe any more. There were some tiny warning signals in my brain. And.....I just don't seem to have got around to renewing my instructor rating, and I don't think I want to. I blame it on no job and no money, but they aren't the only reasons, if I'm honest with myself.
Of course, instructing is very different from flying as a PPL, and helicopters are more demanding than f/w aircraft, particularly when something goes wrong. And this might have happened to me in my 60s, 70s, 90s...who knows? I only know that whereas 10 years ago I would have said that age made no difference, and been outraged at the suggestion that it did, I know now from experience that it does. For me. For now. For the rest of you? I'm not arrogant enough to know, or even to suggest, or to lay down any kind of law or particular age for stopping. But I do think Mary has raised an important issue here.