Is a note from your mother OK?
Talk about a half-ass measure ... Well, it may work 50% of the time.
Kids will just used passed around pre-paid card numbers LOL -- you think teenagers are stupid?
In the USA; Credit Card age verification was found unconstitutionally burdensome on adults with no credit cards in the COPA Act. That was 10 years ago before ATM or VISA/Mastercard branded Debit cards -- who knows -- the Congress may try to rewrite the old law.
The real problem with credit card age verification is adults may not be so willing to expose their data to porn sites -- porn sites have a very bad record with the public on identity security and credit card abuse (banging credit cards). Only the largest and most trusted names in the porn biz may overcome these hurdles. BTW, Ashley Madison:
Ashley Madison's parent company makes $11 million hacking settlement | abc7.com agreed to settle, remember them -- and this was only over non-financial data. Age verification is like buying a porno-perv card in consumer minds -- the government is really trying to make porn less accessible -- this may cost you sales to legitimate adults ...