I don't really look like a teenage boy. I'm 5'2, have a feminine face, and my blonde hair makes it so that even if I could grow something of a girl-mustache, it'd mostly blend in with my face like my arm hair does to my arm. But, I'd like to think that when I'm read as a girl I pass as the 17 year old girl I shouldn't be. Or at least as a teenager.
So, when I get called something that implies I'm a young teen/tween/kid, I'm okay with it. (although getting 'sir'd is a bit odd to me, I never got a 'ma'am' and I'm not sure if I should be glad for passing or confused at the age) What I'm not okay with is that I can't seem to pass when people know my age, since they can sort of see teenage girl behind the preteen boy, so college will be interesting.
"You go here? Woah, did you, like, skip 5 grades?!" "...I'll be 18 in November" "Oh, sorry, *mumbles to friends that they thought I was a boy*" "damnit, you were right the first time >_<" Yeah, joy. I can't decide if that or "but that's a boy's name" when I introduce myself would be worse.