I think I probably look a little older than I am -- I'm 23 and don't get carded nearly as often as I did even a year ago. Part of it's the premature silvering of my hair, I'm sure, as well as a little extra weight that I've put on (so I don't look like quite as much like a skinny young geek now, heh). Also maybe due to wearing glasses more often than contacts these days.
Although I did get one of those door to door magazine sellers yesterday who had a shocked reaction when I answered that I was indeed the man of the house, heh.