Age ain't nothin' but a number.
If I want to fucking act like I'm 10, then I'mma do it. I've never understood the whole "I feel old" thing. To me, it always comes from taking themselves and life too seriously. I play around like and have the sense of humor of a 12 year old, yet I have 2 jobs, paid off my car, going to college fulltime, and I'm poised to move out. (All at the age of 17, mind you)
It's all about how you carry the extra years you get tacked on.
The best advice I can give you is: Lighten up. Go buy some feetie pajamas, Order a Happy Meal, Stay up 'til god knows when, Play video games and talk shit like a kid, do whatever the fuck makes you feel young. My favorite thing is to do driveby cat noises.
tl;dr Lighten up, yo. You're not old until you say you are.