I'm in my 50's and still feel as fit as I have ever been, partly due to the fact that I now have time to run, ride, go to the gym, surf, kite etc.
When I was in my 20's and 30's it was all about the family, mortgage, work etc, at the end of the day I just wanted to sit down and have a few drinks etc.
Now days the kids are grown up, wife has left me (damn what a shame

) I have way more me time and live a much healthier lifestyle.
I fractured my fibula a couple of years back when a kiteloop landing went a
little totally pear shaped so I don't do much of that sort of thing anymore and ride a surfboard strapless 99% of the time but still enjoy learning new stuff i.e. riding waves switch, tack turns, attempting strapless backrolls etc.
I competed in a 20k SUP downwinder on the weekend, Dawesville cut to Singleton (Eppo, Waveslave you might have seen us go past there was a few kiters out) the guy that won the over 50's division went very close to winning it outright, he came in 3rd overall and in front of all the O40's and all but 2 of the open guys.
I can see that somewhere down the track I will start getting a few more aches and pains but so far I am amazed at how well I feel after marathon sessions, but as others have said you just don't take the risks that some of the younger guys take.
I hope to be still improving some aspects of my kiting when Im in my 60's and 70's but by the time I am in my 80's I might have to try something a bit less physically demanding like base jumping or something.