All 'Peter Pan' stuff aside (because most of us have that urge to be immature occasionally for the rest of our lives)....
I think "growing up" , as in maturing, happens when you really, TRULY understand the concept of personal responsibility.
I've been "growing up" in little bits and pieces ever since I left home to go to uni, and over the last ten years it's continued with entering the workforce, renting flats, paying bills and being a member of society.
When you are a teenager - and even in your early twenties to a certain extent - you still don't understand that everything is eventually going to be up to you. Mum doesn't pack your lunch anymore, you have to do your own laundry, remember people's birthdays... if you want that job you have to study for it, if you want those clothes you have to save for them. And if you fuck up your life it's only you who'll be hurt. All those things gradually dawn on you as you get older. The pinnacle is probably when you have kids - now that's personal responsibility to the max.
The good thing I've found is that with this realisation that you're on your own, comes a certain power. If you live your life with no responsibility you also have no power, because all your choices are made by other people. It's a great feeling to think: I'm earning this money, supporting myself, working towards a future, and it's all down to me. When you achieve it, it's all *because* of you, and that's far more rewarding than being handed things on a plate.
Of course there's always room to go out and be silly and immature again to forget it all (for a while
)
Did *any* of that make sense?