I think there is a big difference between the surfers you can call kids who more or less accidently stumble into a porn site, and those in puberty who seek free porn for one of their 15 daily jerkoffs.
If you want to stop teens from watching porn, then remember you are up against one of the strongest human physiological needs, sexuality. Good luck
If you want to stop preteens from watching it, the best way to do is doing nothing except using a filter and a warning. Then its the parents/schools responsibility to limit the kids access by installing the software. If they don't; I can't think of more hypocratic parents that in first place create the market, and at the same time doing nothing themself to prevent their kids from watching it, even if they are offered free software all over. You can compare it with dad buying a porn magazine in the 80's, and not hiding the magazine good enough, and when the kids find it, he blames the producer...dooh!