At what age do boys start acting like "MEN"? What makes a boy a man?

if we were to commit the sin of generalization, at what age do most guys finally stop acting like little boys and start acting like men? My thought is many guys seem to really mature around 28-30. What do you think causes this? What do you think the difference is between acting like a "man" and acting like a "boy"? (my personal opinion is that they start treating women with more respect, they tend to be more self ured and less ego centric, and they stop trying so hard to prove how macho they are).