I'm 24, for reference...
The town I live in is weird; there is an abundance of girls that are 19 and 20 years old, and there's plenty of girls that are 27/28 or so. I've found that pretty much all of the older girls have been here a while and already have boyfriends, so that leaves the younger girls...they're pretty much good for hooking up and gettin your rocks off, but in terms of relationships they just arent what you're looking for. You get the occasional hot tourist chick here and there which is nice...not sure where I'm going with this but
The younger girls I've found just are not quite socially or emotionally mature enough to have a relationship with much substance, thus I don't really date girls who are under 21...it's nice to be able to go out to the bars with a girl instead of drinking at home or wherever