I used to have people think I was slightly younger, not tremendously, but a little tiny bit. What I ended up doing to solve that was I grew facial hair and kept it all the time. I'm 20, and since I have facial hair, and generally have very "adult" mannerisms and all that, people usually always think I'm like....23, I've even had one person think I was like 28-30. One time when I was 18 or 19, I was at a restaurant, and was offered complimentary wine, no carding or anything (I was afraid of getting a DUI, so I declined) just because I came in alone, acted very respectfully unlike most other people that age, and was wearing dress pants and a sweater.