"I'm not sure I can think of a specific crime that anyone under the legal age of consent is committing by running a porn site"
a person under the age of eighteen (in the US, i believe) cannot sign legally binding contracts. if this is so, then they are probably violating a great many contracts if they are an adult webmaster, such as those with affiliates, content providers and hosts.
as far as underage kids having access to porn, i think they should sneak their dad's playboys/nudie playing cards/dirty books like all previous generations before them. these fucking kids got it too damned easy
