The "age" of conformity

A friend told me today that I should grow up. I guess I will forgive her because 1) she's my friend, and 2) it was said in ignorance.
I'd like to know what is this growing up business and why it is so imperative that I do it? Maybe I will be non-conformist and not grow up. Or maybe I have been grown up for a long time, or maybe nobody ever really grows up and anyone who thinks they are grown up is stuck in a sad delusion. I mean delusions where you have fun are one thing but why pick something boring like growing up?