My personal cutoff is more maturity-wise than age-wise, but if I had to put a number on it it'd be 17-18ish, but 18 is the legal age here.
Girls these days though, some of em develop freakin early (hell, ask Kells about it //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif ). Freshman year I knew this really hot chick, and the teacher tried to covertly hit on her and we all knew it. First day of class we had to introduce ourselves, and as she walks up, the teacher stares at her *** the whole time. Then when we read Romeo and Juliet, he made himself Romeo and her Juliet. Someone laughed, and he got pissed so he made the guy who laughed Romeo and some ugly chick in class Juliet. That was some fvcked up but funny shit. Sad part was he was married too.