>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.

Let's try it. If it doesn't help the kids, we can remove it I guess.

Anything that doesn't accept this limitation becomes dystopian. The only viable age verification system is one that cannot be 100% effective, period.