Depends on the laws in your state. In California, an 18 year old can have a sexual relationship with a minor, provided they are within two years of each other. And, that makes sense. Let's say a 17 year old and a 16 year old are in a sexual relationship (which is perfectly legal) and the 17 year old turns 18. It doesn't make sense that the 18 year old would then be guilty of statutory rape, when nothing about their legal relationship has changed except that someone had a birthday.