I had sex for the first time when I was on the older side of 14. It probably wasn't old enough, but I'm still glad it was my own experience and that nobody else was involved, i.e., no parents told (and I say this as an almost 40 year old responsible professional adult). But even if it was okay for me, I don't know of many others then or since for whom it would have been. All of this is to say, 13 is too young. Without knowing more though, I don't know the best course of action. I would definitely try to get closer to it to figure out how to help.