Hell, grow a beard .
I had the problem the other way around. Even when I was 16-17 years old, people found it hard to believe that I wasn't over 20. Especially since I've grown a mustache ( got rather attached to it too ) that stuff happens damn often. At least I never had any problem buying alcohol when I was still underage
You actually do look a little young (not twelve, mind you, it's prolly just an age that people commonly use when making such comments) but you shouldn't take it too harsh. Maybe people are actually trying to make you a compliment. If you really care so much, grow your hair and grow a beard, try the look, and if you like the results, keep it.