I'm 22 and still get asked if I'm old enough to sit in the exit row or enter 18+ venues, let alone 21+ venues. It's a complement. Oddly enough, people think I'm younger now than they did when I was 16 and relatively skinny. I'm 6'1", so I'm not exactly small.

My 16 year old brother is 6'5" and is a lineman for his high school football team, so he's not a small person by any means. When he was 8, a ticket agent put a note in that the pilot should consider him as an adult for load calculations. While he's occasionally asked if he's old enough to sit in the exit row, people often think he's the 22 year old and I'm the younger brother.