Legitimate businesses can get a lot more failures to conduct business, since customers would not always have the card ID ready at hand.
That can be handled with an awareness campaign analogous to the We Card program, building an expectation among citizens that if you're going to subscribe to porn, you're going to need to have handy a means of age verification.
just asking the question above does not create a bunch of opportunity for credit card fraud.
But it does create a bunch of opportunity for the government to shut down your business on grounds of not taking legally sufficient precautions against underage access. It's a balancing act of liability for credit card fraud vs. liability for fines for noncompliance.