Haha, I know this feeling only too well! I'm a 5' tall Asian, people guess me for 4 or 5 years (at least!) younger than I really am all the time.
The first time I went to a casino, I was 22. The security guard asked me for my ID, scanned it with one machine, looked at me, scanned another machine, looked at me again, and finally handed my ID back and said I could go through. You could just tell he didn't think I was really old enough! I wanted to say "I promise its not fake, I really am over 21!"