"EWWW your dating a little kid!" "umm no, he's 15!!!"?

I am in grade 11 and honestly look like i am 12 years old. I am actually 15...but whenever i tell people like that they are like WOOOOWWW!!! ive gotten introuble at school twice for being in a "grade 11 and 12 area only" ahhhh. I have a new girlfriend (we've going out for 2 weeks now) and we usually always hold hands. when we walk past people she knows, or is friends with...they are like "HOW OLD IS YOUR BOYFRIEND! :O" and shes like...."our age...15!" and they get surprised. It happens EVERY single day...and people at school stare...and keep staring!!! OMG! It really upsets me, cause my voice is not the deepest either. Someone came past the other day and said "you look like a FCKIN TEN YEAR OLD" to me, and that kinda hurt :/ I know it annoys my girlfriend as well. ive been asked more than once if "ive hit puberty yet" it kinda gets me down...and i feel bad about myself. I know there isn't anything i can really do, about my apperance...my body is really developing itself slowly grr...but what can i do about the whole..."EWWW your dating a little kid!" "umm no, he's 15!!!" incidents?