The offense is dependant upon the age of the actors. In some states, the age difference can also be a factor. A person younger than "the age of consent" in that state does not have the legal capacity to give consent, even if they are a willing participant in the act.
On the basis that the teenager is not capable of consenting. That they are not developed enough intellectually to know what they are doing. Of course cases vary widely, and the law is a blunt instrument, in many cases used by a blunt-brained people who are often confused about sex themselves.