When I was younger, like kid years and teens, people always seemed to think I was at least a few years older than I really was. I don't know how much older exactly, but it did lead to quite a few dubious looks. "Yes, she really is x age, believe me."
Of course, now that I'm older, people have mistaken me for someone in my teens at times. I've had several people ask me if I'm either still in high school, or have guessed my age as 18.
...Neither is anywhere close to correct.
I don't even know why they think that, they just do. o.O Although I've always been kind of tall for my age, especially when younger, so I could see them thinking I was older then. I can't explain why people think I'm younger now though.