I've been in touch with a few porn hosts/webmasters on story ideas and have found real reluctance to post things in the mid-teen range--never mind young teens, pubescent, and pre-pubescents. There's a paysite, http://www.mrdouble.com/ that seems to have a lot of adult/young teen sex stories, with a fairly high representation of nc, violent, and--especially--incest. I don't know that for a fact, but that's what I gather by poking around their rather sparse free material
For whatever reason, I am very attracted to extreme nc/sm/bdsm content involving girls (especially the prim and proper types!) ages 13-17. Then I have a fetish for virginal brides, but that's another issue.*
It seems that the sexual revolution has now come to full flower, resulting in the deflowering of kids at earlier and earlier age. Now, if a certain percentage of us older folks are attracted to this kind of sexual activity, then it follows that a similar percentage of the teens would, too. Perhaps more! With the increase rate of broken homes, single-parent households, domestic violence, etc. going on, then it's a reasonable assumption to think that these dysfunctional relationships among the younger teens will result in dysfunctional sexual expectations--and practices.
The important thing to remember is that all this stuff has to be consensual if it is actually performed. If it is not, then it has to be strictly fiction! The way political and moral trends are going, I don't see the "age of consent" being lowered anytime soon. It may be that a "de facto" lowering may occurr: sex among younger teens will be tolerated, as long as it doesn't create a problem, but will be acted upon if it does (pregnancy, delinquency, etc.) Then adult-younger teen sex might follow. For example, many areas have "de facto legalization" of small quantities of marijuana. Enforcement is rare, except for when it is part of another problem (say, a disruptive person or a car accicent). Then they arrest you!
But regarding the posting of young teen sex stories! As long as there is a proper disclaimer, I don't think anybody is going to get in trouble there.
*If there is anybody interested in this unusual theme, please respond to a thread I started, currently on Page One, titled "Briderape." I'm surprised that nobody has picked up on it!