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 (boxing, muay thai) and I don't need as much sleep as I used to when I was a kid. You don't want to step in the ring with a guy in his 20s/30s because your reaction times are ~80-150ms slower but that has very little impact on your ability to change a diaper.
Step outside, touch some grass, and meet people in the real world.
I have a 1 year old toddler at home weighting 12 kg, so thanks, but I have a direct experience of it. It's a pleasure to be a father, but yeah, lifting him, the stroller, waking up at night, while having to provide for the whole family isn't a walk in the park physically.
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...
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 (boxing, muay thai) and I don't need as much sleep as I used to when I was a kid. You don't want to step in the ring with a guy in his 20s/30s because your reaction times are ~80-150ms slower but that has very little impact on your ability to change a diaper.
Step outside, touch some grass, and meet people in the real world.