Never.

But I have been mistaken as someone younger than I am. Last year (I was 21) someone thought I was 15 and rudely asked if I wasn't too young to be drinking coffee.

I don't mind too much, except when it interferes with the business aspect of my life. People tend to shy away from doing business with people they think look like they're in High School. But I figure in the long run I'll appreciate looking young for my age.