I hate it when people say this to me. Especially people at work. I introduced myself to a new woman at work the other day, I told her what my job was, and she said, "Wow, you look like a kid!" Then she saw the cool look on my face and immediately said, "Oh, it's a compliment!" And I said, "Whatever, bitch." Just kidding, I didn't say that.
I suppose telling someone "You look so young" is slightly better, but even that is annoying, especially at work. I mean, I don't mind getting carded when I buy drinks (in fact, part of me kinda likes it), but at work I think those kinds of comments are inappropriate. This is a sore spot for me because I've always looked young and felt like people didn't take me as seriously.
I started taking Spanish a few weeks ago and my teacher looks pretty young. I thought she was in her mid 20s. During our first class she was explaining the difference between senora and senorita. We asked what should be the default when you don't know if a woman is married or single. We assumed senorita because that might be flattering. She said no, not necessarily. She always prefers people to address her as senora (even strangers) because she's in her 30s and married. She also wants people to call her ma'am, not miss. I don't feel strongly about being called me ma'am, miss or mrs., but I guess everyone is different.