I'm 26 and currently seeing a 19 year old.
I think her being under legal drinking age is the big factor. I've fvcked a lot of women at bars... young ones, with boyfriends. It amazes me how many women want to have sex with a stranger or have a one night stand. Point is, these girls who cheated told me these things, and I think they're common desires of young women and alcohol allows them to do these things even easier.
If the girl is young, she needs to have real world experience in my opinion. I'd rather sleep with a 18 year old who's done some things sexually that have allowed her to experience men than one who hasn't, because I think she's going to want to either way. If she's 18 and hasn't experienced the bar scene, then she's going to be jealous of her friends when they're 21 and doing it and snapchatting it and crap.
Girl I'm seeing lived in Europe for a few months, I'm sure she took some ****, but she's seen the bar scene and screwed a stranger and had a one night stand and all that. I'm kind of thankful that she has because she knows (or at least think she knows) what she wants in a relationship.
This said if she's young she's able to be shaped into whatever you want really. I think a relationship with a young woman will be incredibly fulfilling or absolutely hellish.
Food for thought.