IF the kids want to look, they will find a way. Nothing stopped me from looking at porn on the internet when I was a kid... I knew everything about how the computer worked and my poor helpless mother was lost :)
Recently I sat down with my nephew and had a talk with him about internet porn. He is of the age now where he has had a couple of girlfriends etc (He is thirteen, fourteen soon). I asked him if he had watched any porn online, and he said that he had. The point of the conversation was to make sure he understood the difference between fantasy and how sex works in reality. Kids are smart, and will engage in a topic if you are up front with them about it and stay non judgmental. He asked me some questions about sex, and what the differences were between the real thing and some of the videos he had seen. I just explained clearly that porn is acting, and not to expect what happens in porn to be the way things turn out in real life. He had made some assumptions about the physicality of things that I set him straight on, and by the end of it I think he had a better idea of what to expect from sex and what it really is.
Also, I feel it important for kids to understand that there is nothing wrong with their urges and wanting to have sex. He knows know that if he does do it he has to be responsible, and understand that they are doing an adult thing so they need to be prepared for adult feelings and consequences.