Most Helpful Guy
Oh, believe me, I share your pain. I just turned twenty-one yet could easily throw on my old school uniform and pass for a fourteen year-old. Being short and having a "baby face" doesn't help.
Save with expensive plastic surgery, you can't really change your physical appearance much beyond a new haircut and maybe a bit of make-up. So what can you change? Your clothing. It plays just as big a role in projecting yourself to the world as anything. If you can't look your age naturally, then the only option left is for you to DRESS your age. If you dress like a twenty year-old people will treat you like a twenty year-old, but if you look more classy and put together then people automatically put you on a higher pedestal. Leave the flaky younger fashions behind and embrace the timeless styles instead, and you should do fine.
Hope this helps :)