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.
If you believe everything you read, better not read.