nation's sex offender registries. estimates suggest a quarter of them landed there before the age of 18. what some of them were convicted of may surprise you. >> if i could go back in time, i would stop myself. i know there's other people in school having sex. i'm the one that got caught. >> tonight we explore a question that should be on every parent's mind. when does teenage sexuality cross the line and become a crime? >> do your job, follow the law, and find him guilty. >> where does it end, and whose kid is next? ♪ ♪