Earlier quoted context omitted.
Good luck lifting 50 times a day a 12kg todler while being in non-ergonomic positions and waking up 3-4 times during the night while having a productive work when you're in your 40's. And then, when they grow up, to keep up with them during physical activities, to teach and show them the way. It's possible, but it's much easier when you're in your 20's.
How old do you think 40 is? My god. Have you ever met someone in their 40s? Are you a child?
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.