If you are 18-years-old and have consensual sex with a 15-year-old, you cannot be convicted of Rape in the Second Degree.  There is an affirmative defense for consensual sexual intercourse between people that are within 4 years of age from one another.  However, you can still be charged with Rape in the Second Degree if the person you have sexual intercourse with lacks consent for a reason other than age.