This Wont End Well
I think this is going to get clobbered by the law of unintended consequences.
1: Teens are going to look for porn.
2: Find out that all of the legitimate sites wont let them in without a credit card.
3: Go and find stuff that comes from providers that don't give a toss what the BBFC says.
4: End up seeing all of the really nasty / illegal stuff that lurks in the Internets gutters.
Net effect is a massive upswing in viewing / prosecutions for extreme porn!
Trebles all round in the house.