I'm 21. People definitely do think I'm younger than I am though... I had my waitress the other day ask me if I went to high school with her. When I told her I was a senior in college, she kind of jerked back a little bit.
I also got carded for asking a bartender for a NON-alcoholic margarita, because I don't drink. But apparently you get carded when you don't want alcohol. Oh wells....
But I'm sure it will come in handy one day...