You want to let every merchant I swipe my card at know my age? To improve privacy?

No, the assumption is that you must be 18 years old to apply for a credit card. Surely we could have the machines determine that an "authorized user card" does not guarantee 18+ but the actual card does.

They don't need to know the exact age. But if kids couldn't get credit cards... then possession of a credit card would be a proof of adulthood.

An absence of "this user is underage" flag would be good enough. Less than 1 bit of private information leaked by transaction.

ceejayoz> You want to let every merchant I swipe my card at know my age? To improve privacy?

Remember the site guidelines:

SG> Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize.

The obvious solution is instead of "every transaction comes with the user's birthday", the vendor can in some way set a minimum age enum of say (13, 15, 18, 21, 25) — a handful of ages that are significant with respect to some law or regulation. Then the transaction succeeds or fails.