I also have this funny thing where I can't build endurance. I've been doing push ups on every work out and I can't for the life of me do more than 20. But I have no trouble increasing difficulty (putting hands closer, elevating feet, etc.) or weight (for things with weights like squats).
I had pretty good endurance in my 20s, but noticed a huge boost in my early 30s.
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.
> The typical social script is to condescendingly explain to them how they must be doing it wrong
This is a bad situation when the person has a problem. I have family with chronic fatigue (+chronic pain) and it's mentally hard to get how they feel, but I've learned that I cannot have the same expectations for them as others.