Open Question: Can someone be a certain age forever?

I know you have to grow up (chronologically/by time), but do you have to ACT your age? I mean, lets say someone turns... 21 or any mature-by-law age... can they still look like and act like that age? Would anybody know the difference? Is it annoying? I'm 15; and I don't wanna grow up at all. If I take care of myself, could I surpress aging for a while? Being mature is just not in my