At What Age Did You Become Less Physically Capable?
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There are outliers, like the 75 year old who passes you on a hill climb. But he's probably much slower than he was 30 years earlier too.
Another limiting factor is you get more "injury time". A knee injury that the doctor said would get better in 6-8 weeks "if you were twenty years old" mostly got better over the course of a year. But that's a big chunk of time.
All this being said, if you enjoy doing a thing, you'll still enjoy it if you have to go slower or not as long. Just do it.
late teens to mid thirties, top physical form
mid thirties, losing the proverbial step (pros retire at this point)
mid forties, it's obvious you can't keep up with the youngsters anymore. Even your superior experience can't bail you out.
mid fifties, it's obvious that if you don't quit you'll be seriously hurt.