Beware unintended consequences
Banning recreational drugs created a black market where dangerous unregulated products flood the market. The same happened when alcohol prohibition was in force.
Make it too difficult for teenagers to access "normal" porn sites and they are quite likely to gravitate to P2P, newsgroups and Tor sites where they are more likely to come across the more extreme and illegal material. Not to mention the circulation of DIY porn made by the teens themselves,
I'm also concerned about exactly how the age of the user is going to be checked. Requiring the user to give CC details to every porn site is highly likely to result in an increase in the amount of fraud.
Biting the hand that feeds IT
