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.