Teenagers have always had sex. It's really important to take on board the fact that in the Victorian era (as a for instance), only middle/upper class people wrote.
So, all our perceived knowledge of eras gone by is by chroniclers who were conscious of their readership .. ergo other readers; literate people of a certain class or aspiration.
They did not want to read that twelve year kids were fucking in the hedgerows at harvest festival, so it was not reported.
But they were, because some chroniclers slipped through the net. People are people, kids are kids, sexuality is sexuality. Many things change, but people never do.
Kids are no more promiscuous now than they ever were.