I'm the same age as you, except I get mistaken for being a twelve year old, both on sight and just when people hear my voice. So, hey, at least they didn't think they were older than you and refuse to play with you because of it! That said, it might not be height or facial structure so much as the way you dress and your body language- I still wear clothes from when I WAS twelve on occasion, because they still fit. I know that doing so confuses people further (and even got me mistaken for a nine year old once... which is a bit WTF...) so I don't wear them out of the house much. Look over your wardrobe and see if there's still a lot of clothes from when you were in high school, or that look like something you would have bought in high school.