I can tell you this thought has never crossed my mind lol. I'd like to look younger! Must be a guy thing. I'd say try not to dress too young, have a little scruff maybe and most importantly think more mature about life.
I've always looked young too. Heck I just turned 30 and I get carded for buying alcohol and told that I look like I'm 16. Honestly I take it as a badge of honor; eating / living healthy and looking young isn't a bad thing. I guess if you really want to look older you could take up smoking or drinking and not shaving. It'll make you look aged pretty quickly. Or meth; google faces of meth that makes you look aged pretty quick supposedly.
But in all honesty I don't condone doing any drugs or trying to look older. Enjoy being young while you are. Plenty of time for looking old when you're older.
If you're a guy, facial hair mainly, put on a nice dress shirt, gel your hair a little, wear a nice watch, shoes etc. Females, dress classy, do your hair, stay away from rad styles like spikes and whatnot. Just look like a woman, not a pre-teen basically. And don't wear enough perfume/cologne too suffocate an elephant