Is it at all possible to totally extinguish one's inner child?

I just don't feel like I have a chance with my crush unless I come across as more sullen, or dark, and all that. I'm younger than him and I'm totally a kid inside. I mean, I have a lot of friends, and I get good grades, so in theory there's nothing wrong, but I guess I'm just too youthful to be worth something to him? I can be hyper or giddy at times, and I've never been one to want to grow up too fast, and one of my friends told me that the only difference between me and his girlfriend is that she's "mature". I'm not a troublemaker or cl clown, and I'm actually quite a people-person and diplomatic but... how do I stop being so young at the surface? I've been trying to be more discreet about my likes and dislikes; one of my friends teases me lightly every now and then that I "STILL listen to The Naked Brothers Band", and I'm still a virgin, and all that kind of petty stuff.