I've always been taken for being way younger than I am. I'm 27, but neither act or look my age ^^ Guess it doesn't help I hang out with people about 7-10 years my juniors...
It has gotten me in some interesting situations before though...
When I first started dating my boyfriend (mind, we are the same age, only 4 months apart), at 17 years old, people kept sending us very weird looks. It was first when a friend asked if it was even legal what we were doing that we found out people thought I was 13 and he was 25 xD