When I was 22 people thought I was 17. Now I'm 29 and people think I look 18. :/
Apparently, it all depends on what kind of lifestyle you live, the people you hang out with, etc.
Take for example a lady I know who's in her mid 30s, but she looks amazing, she doesn't look a day over 30, she doesn't drink, she doesn't smoke, she doesn't go out partying a lot, she doesn't hang out with the wrong crowd, etc.
Then take people like Lady Gaga and Rihanna... it ALWAYS surprises me that they're both in their mid 20s, because they both look MUCH older than that, but then again, look at their hard-partying lifestyles.