When I was eighteen, nobody had any idea how old I was. People thought it varied from ten to twenty-three (I know, huge difference for that age.).
I'm twenty now, and I can barely even pass for the 12 and under rates for movies. I hear 15-19 a lot nowadays for my age. I've filled out a lot more since I was eighteen so that helps. Plus I'm mature for my age. The only things holding me back is that I'm petite and when I stand next to my 5'8" mom, people automatically assume I'm younger rather than when I'm by myself.
What I really hate, though, is the shocked look I get from random people when I step out of my car, like I'm too young to be driving or something. I understand that look from kids, but not adults.
I've also noticed that the older the person is, the younger they think I am. But if they're in my age range, they know better.