Well when people automatically start treating you like a man because you act like a man and people acknowledge you as being that, you are a man in my eyes. It's a fluid transition that can happen suddenly or take years. I think the reason we have so many different events set for when a boy potentially becomes a man (like turning 18, getting married, moving out from home, being independent of your family) is because a lot of changes will occur for a boy/man at those times that can change their personalty and circumstances or appearance, to the point where he might change enough to become a man in the eyes of the subjective community around him.