If a guy asked me, "Are you sure you're over eighteen?" I'd feel perhaps a little miffed that I looked so young (with connotations of immature), because normally people assume I'm in my early twenties, but also be pleased that he was checking. However, I might worry that the guy had had dealings with underage girls before that had come back to bite him in the bum. So I'd be happy to prove how old I was, but I'd want to know more.
Yeah! I mean, I wouldn't go home with a man I'd just met in a club, but say I was in a position to be doing that, and he asked just as we were leaving, "You are eighteen, aren't you?" I'd get a few warning bells going off as I don't look particularly young. I'd probably jokingly ask, "Why, have you had trouble before?" but I would want an answer.
If the guy had had underage dealings and got in trouble for it, I'd probably be rather put off. If 'a friend' had and I believed it was a friend, okay. If he explained he was just being responsible and that I didn't look like a twelve year old and I believed him, also okay. And I definitely would appreciate and respect that he made a point of asking! Guys and girls both have to step up and take responsibility for their safety and their actions, but I do think it's guys who are most in danger of accidentally picking up a girl who is either legally too young for them, or just below what they feel is acceptable for them.
Oh, I don't see it quite that way. I've no problem with people who want to indulge in one night stands, as long as they're safe and sensible about it. It's just not something I want to do myself.
My point was perhaps more about how, whatever it is you're doing or planning on doing with a guy, there are certain things that they can say or do that set off warning bells in your head and require further explanation.