That's not completely true. There are clauses in age of consent laws for when the individuals are close in age, but someone who was under age can still be charged and found guilty of sexual abuse.

One of my abusers was an under age babysitter. By the time charges were laid and it went to trial he was in his late 20s or early 30s. He was found guilty and sent to prison for a time.

It really depends on how it was being initiated, whether she had any authority, how much older she was, etc. Depending on those variables, the fact that she had been going through abuse herself would lend credence to the argument that she was engaging in abusive behaviour, if that argument was to be presented. My sister reacted to our own early years this way, becoming an abuser herself towards me through our childhood.