When is it that young men start to grow up to be real men?

Back again? Well IDK what to say my growing up came from knowing I had to get things done. Then watching history channel and then i got addicted to learning.

Break up with him and see if he matures. If he does not find then find a better man.

I think they start to grow up in their 40s.

It's true girls mature faster than guys, but I think many of them catch up fast. I don't know, guys just crack me up. They could have lost their job, their wife, their house, their car and all their cash, but they can play a game of pool and have a few beers with a mate and not say a word about it. If you ask them why, they will just say "He didn't ask."
I think true maturity is knowing that no-one can solve your problems or make your life better but you - and most guys I know seem to know this - which I guess, to answer your initial question, is why they are men and not boys.