Maybe STI testing if she hasn't been practicing safe sex every time, all the time.
YMMV, it doesn't sound like your daughter is heavy into rebellion, but you don't want to get into the whole forbidding/I can do whatever I want! That usually creates a motivation for the child to act just to make a point to the parents.
Also, try to validate her feelings on this - I suspect she finds his "maturity" attractive, let her know that you understand that and admire that she values maturity. I think it would be important to express that you feel finishing school and focusing on college should be a priority.
I've been a lurker here for some time, and I've seen some great advice given out. I found out that my 17 (18 in 6 months) year old daughter is dating a 25 year old guy that she had previously only been friends with, and Im not sure how to handle this.
First off, my Daughter is a smart, funny, mature, independent girl with a lot of friends and who does very well in school, and is preparing for college next year. She broke up with her last boyfriend (who was her age) because he always had drama going on around him, and unlike a lot of teenagers I know, she hates drama. ) Daughter met him some months ago when her friend got her uncle (The Guy) to drive them to the mall.
Trust your daughter's opionions, but talk to her to make sure she knows you're there for her and that she needs to should stay strong. I'd try to find out what this guy's other friends are like. Ditto on I don't think that anything you do or say will change her mind toward him, but will drive her away from you.
You want to make sure that her relationship with him doesn't derail her future college plans.Keeping your relationship with her as good as you can is about the best you can do.And you might also ask her how she would feel about a male (or female, I guess) friend her own age dating a 12- or 13-year old (not the same difference in years, but IMHO you change just as much in those years as you do in your late teens and your twenties).A teen can spend six months in a bad relationship to avoid having an awkward conversation explaining it to their mom.I know it seems impossible, but I think you want to treat the relationship as casually as possible.I suspect she's not as level headed as you think she is.