What makes you think this will be close to 90%? Unless these cards are expensive I don't see that happening.
its obviously just an illustrative guess. but if the penalty of possessing the card is similar to underage possession of alcohol/tobacco, and larger penalties if a store/person is found providing a card to someone underage, i see no reason why it wouldnt have a similar success rate as alcohol/tobacco.
hopefully some parent steps in if their kid is on the dark web trying to make purchases with their parent's credit card.
Anything that doesn't accept this limitation becomes dystopian. The only viable age verification system is one that cannot be 100% effective, period.