When I was in my late twenties, a few high schoolers asked me what year I was in at school and they were highly embarrassed and apologetic when they discovered what age I was.
My brother saw my year/grade 8 photo and exclaimed "You look like you're in preschool!"
One girl told me on my 16th birthday that she couldn't believe I was 16 because I looked like I was 12.
Everyone has always assumed I was younger than my younger sister.
One guy told me (when I was an adult) that my mannerisms make me look young too.