Here in NY, the difference in ages can be used for what is called an "affirmative defense". Which is to say, that if a 15 year old is having sex with a 17 or 18 year old, because the 15 year old is not yet the age of consent and the other person is, the older person can still be charged with a crime, but can use the age difference between the two parties as an "affirmative defense" and charges are generally dropped. In NY, the age difference must be less than 3 years, 9 months I believe.
I work in foster care (as I said, sorry to repeat) and I was told by a work at another agency that a 16 year old boy on her caseload was currently in detention for having sex with his 15 year old girlfriend, even though it was consensual, and even though the parents of the girl were aware and supported the relationship. There was a feeling that an disapproving friend or school administrator had made some sort of call. They expected the charges to be dropped because of the age difference, but in the meantime, this kid was in JAIL for having sex with his girlfriend.