The above-18 person gets charged with rape, even though the minor partner gave full consent. The minor partner gets grounded for life, if not juvey.
Now, for those not in the know, PLEASE do not take this issue lightly. If you have sex with a minor, it is, without the slightest of exageration, A CRIMINAL OFFENCE. Even if it's a day before the minor's 18th (or whatever) birthday.
Lots of kids are unaware of this.
And don't think if you're a minor, you can get away with having sex with another minor. There's a place to detain juveniles as well.