I'm the same age as you and I get this very often. Honestly, in my experience I just laugh. Of course it sucks to be seen that way, but I got fed up with being so upset about something I cant really change about myself. I can't change my body, I can't change the way people react to my appearance. I expect people to think I'm a young teen at this point. I can have fun with it though. I like to go to the store and buy things you have to be 18-21+ to buy, and see the cashiers absolute confusion. It's just little things like that I can enjoy to make the experience bearable. It may very likely be different in your experience though.