I suspect it was population dynamics and the values were arrived at by trial and error(luck).

For a population to sustain itself enough children have to survive long enough to mature and be able to replace themselves -- but not so long they become burdens. Whatever best age to support population survival will tend to hang around.

And of course there is lots of interplay. A little bit of variety may bring advantage.
We've had hundreds of thousands of years to mix and match our people to try different things. Unlucky experiments die off and successful experiments survive to procreate.

Eventually The Gamblers' Ruin(tm) will get us. We'll have a string of bad luck and our population will drop to the point where we don't generate enough successes to keep going. Then the game(we) will end. The house always wins and the players always lose. I think we've had a pretty good run so far but it won't last forever.

I'm personally scared we may be approaching that point now.
I never dreamed I might live to see even the beginning of the end. But it's not like it matters much. We won't live to see how it turns out.

Fairy tales don't always make sense.

There is a Greek fable that when the gods created the animals they offered them all fifty years of life, the donkey did not want 50 years as a beast of burden, so thirty were taken from him, the dog did not want fifty years of growing old and pain he just wanted his youth, so forty were taken from him, but when man was offered fifty he said that was not enough, so god gave him the thirty from the donkey to have as a time to work hard, but man was still not satisfied so god gave him the forty from the dog to play in his youth and rest in his old age.
Man however was still not satisfied so the gods gave him a mind and imagination to try and find other ways to extend his life for himself.

Maybe some of these ages were just typos? The scribes weren't mathematicians.