I'm always mistaken for younger than I am.
I have had my ID rejected because my birthdate says I'm 33 and the person looking at my ID couldn't believe I was that old. Door workers at bars and clubs often look at my ID, look at me, look back at my ID and say, "No way!" People guess I'm 25 most of the time. In college, people always thought I was a freshman, even when I was a 23-year-old senior. (Took time off.)
After graduation, I began subsititute teaching a lot at a nearby elementary school. One day, a teacher there stopped me and asked how I managed to get "so much time off school." I said, "Oh, I graduated in May." She said, "That explains it! Are you going to college in September?" I said, "No, I mean I graduated from Smith College in May." She did a double-take, began to laugh, and told me the entire staff thought I was a high-school student who had finished all the high-school courses a little young. At 23, my co-workers thought I was 16 or 17.
I still get carded every so often, though there's no possible way I look that young. I always feel like they are teasing me!