Sexual offenses such as statutory rape involve minors. This is a type of sexual abuse specifically targeted at those who are under 16 years old. There are a few exceptions if the other party is within four years of the alleged victim's age -- such as a girl of 14 and a boy of 16 -- but the rule holds true for many cases involving minors.
Of course, not all minors are going to report incidents on their own. This is true not just for statutory rape, but for other types of sexual abuse. This is why the law requires that some people make these reports on behalf of those minors.