I have the opposite problem, in that I look older than I am. When I turned 21, I went into a store to buy a 6 pack of beer as a "ha! I'm 21!" thing. The clerk didn't even bat an eye, just rang me up. Kind of disappointing, as I wanted to show off that yes, I was legal. If I had known they weren't going to check I would have started buying beer earlier.
That whole year, I only got carded about half the time I went anywhere/did anything.