It's a continuum. Two years ago (at age 55) I made an effort to get into decent cycling shape and certainly was the fittest I'd been in a decade. But comparing

It's a continuum. Two years ago (at age 55) I made an effort to get into decent cycling shape and certainly was the fittest I'd been in a decade. But comparing to similar level of fitness 20 years earlier, I was still 10-20% slower in every respect. I say "was" because I've backslid since and aged another couple of years. I'd say you can maintain peak physical ability until about age 45 and from there it's downhill. 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.