This is a real sore spot for me. I look a lot younger than I am, and no one can explain why. When I was in college the city bus driver assumed I was in middle school. I don't think I act young nor am I short or small, or dress young, etc. Even at work people comment on how they can't take me seriously because I look like a teenager - despite being dressed professionally, etc. When I went shopping for work clothes (in my mid-20's), the sales lady asked me if I wanted a gift receipt. I told her they were for me, and she was shocked, saying someone my age should shop in the kid's section. Like I said, I'm not short or small so I don't understand why people say these things. I think MAYBE it's because I'm so nervous in public because of anxiety that I give off the impression of immaturity.
I think the worst part was when I was pregnant and in the hospital, I overheard a nurse talking about how it's a shame kids are having babies younger and younger, and how "the girl in that bed (ME) can't be more than 12 or 13" - yeah, try adding more than a decade to that.
So I usually am all too happy to tell people my real age, and then brace myself for the "I never would have guessed" speeches.