Bro, don't listen to anyone! You look ur age. Try maybe spiking ur hair it will look better on u, and also start going to gym, or maybe start swimming. Swimming works out lots of muscles.
Probably not 12, but that's certainly not a stretch of the imagination by any means, I think it could swing either way, 11 even to 14. No where near 16, hope that's not offensive or anything.
Its good to look younger than you are, when your older..
Right now, I can't exactly judge your age because I'm terrible at that stuff, but your just a small guy man. Your skinny and your arms are a little small.. maybe that throws off your age.
I'm a bit bigger of a guy, and people still think I'm younger than I am.. well, sometimes. Some people think I'm older.. others younger.
Hit the gym, you're too skinny. And Thelis is right, it's about time to grow up and realize that no one cares how comfortable your clothes are, they only care how they look. Not that a nice shirt isn't comfortable...