21 year old male that looks 15...siiiiick of it?

It's not that bad, it's normal. Eventually, you'll look your age. Until then, just act like you feel, or at least, act your age. Even if you look 15, it doesn't mean you can't be 21. Look at Sterling Knight, TV star. He's 20 and he looks 15. He currently plays a teen in "Sonny with a Chance" and played Alex O'Connel in "17 Again".