One interesting data point is to look at the age of UFC champion for your weight, who is probably one of the most fit people in the world.

However, you need to (sadly) keep in mind that weight-cutting is a thing and compensate for that.

So, for example, if you weigh, say, 175-210 pounds, presumably you'd be in either Welterweight or Middleweight.

The champions of these divisions are 34 and 37, respectively.

Of course, they cut a substantial amount of weight, which favors younger fighters. It's possible they'll be at their max fitness in another 5-8 years, even though their capacity to cut weight will have diminished.

There's a message in the zeitgeist that max fitness happens in early 20's, but the evidence, at least in the martial arts world, is that it's late-30's to late-40's.