>But if someone is willing and able to pay

If someone is willing and able to pay, they have a source of money. If they aren't allowed to buy something, that control should be applied at the level where they get the money. If the child is using an adult's credit card, responsibility lies with the adult. If children need to have their own credit cards, the obvious point of control is the credit card itself.

But also, most porn is ad-supported, pirated or free. Directly paid content is a small fraction. So all of this is moot for porn.