The athletic landscape is changing. Guys like Tom Brady, Justin Gatlin, LeBron James, the big 3 in tennis (Federer, Nadal, and Djokovic) have all shown us that when you prioritize your training demands appropriately for your age, and place a premium on intelligent recovery, that you can maintain even Elite level performance for a very long time, into your 30s and 40s.

And that's at a level of performance where millions of dollars and GOAT status are on the line! But if you're just talking about being reasonably athletic I see no reason why you can't maintain that for much longer, potentially into your 50s and 60s as well. By being smart and adapting/evolving your methods accordingly as you get older there is absolutely no reason why anything has to start automatically going downhill just because you have crossed some arbitrary age threshold. 

It has become more and more clear to me over time that physical attributes deteriorate with age not because of the age itself, but more so due to accumulated orthopedic issues that prevent adequate hard training (in dedicated athletes) and shifting priorities (nice way of saying some people just quit as they get older).

But you don't have to become a washed up meathead (if you don't want to) who yammers on incessantly about "well back in my day I could blah blah blah" to anybody who will listen. Personally, my goal is to be as athletic as possible for as long as possible. Maybe my 40s will require another training evolution for me, and my 50s another beyond that, buy by and large I think I'm on the right track.