Can't use a credit card to verify age. Think about it.
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You are not comparing the same things. If something counts as an ID, it will proof you are, who you claim you are. Using it to verify your age is only done as a side effect and mostly in the country where you can buy alcohol at 21.
You can't get a credit card if you are a minor. Sure, you can borrow one from your parents while they are not looking, but you can also borrow the ID of your older brother that looks like you and buy beer with it. Or just get a fake ID for that purpose or ask some friendly neighbourhood bum.
Actually using your parents credit card as a minor might even provoke some needed parenting, since those things will appear in billing ;-)
Anyway, having access to a credit card can be used online as plausibility check, I would not like it. It would exclude a lot of people for a lot of reasons. And any minor that would want to access any content behind such a wall, will just access it by other means and probably will know more about how to do this, than their parents. It is similar to drm. It does not really protect, but will harm the legit customers. Also, in regards to recent events, giving credit card companies even more relevancy is not the right thing to do.