Earlier quoted context omitted.
40 is older than 20. You then don't have the same resistance to stress, lack of sleep, muscles regenerate slower, and you often start to have health problems. Believe me, most parents have back problems at some point, especially if you're not sending your child to daycare. No wonder, armies around the world won't hire a 40 years old, and prefer youngsters in their early 20's: their body is still intact.
If you think being a parent is as physically grueling as combat then I'd suggest you either meet some parents or get into some fights because you have a serious misunderstanding of one or the other. In any case, outside of the extremes there really isn't much physical difference between 40s and 20s unless you have health issues or you don't exercise. I'm 45 and lift about what I did when I was 25. My cardio is better…
I'm really happy for you that you are entitled with enough time and money to lift/box/muay thai/cardio - I personally have to keep the bank account loaded enough to make ends meet, while I'm not at home helping with the daily chores. I did a lot of sport in the past though, I was a competitive rower in my home country, and I kept a very strong cardio out of it.
Anyway, why not a bit of skin in the game and test your theory about "no physical difference between 40s and 20s"? For instance, by marketing a health insurance in the US, where the premiums are the same for this age category? I hear that it's a good business, so why wouldn't you try? https://www.valuepenguin.com/how-age-affects-health-insuranc...