Prove of adulthood should be provided by the bank after logging into a bank account. I'm sure parents just would let their bank details be stolen and such.

Of course no personal details should be provided to the site that requests age confirmation. Just "barer of this token" is an adult.

The "Bank identity" system in Czech Republic (and likely other countries) can be used to log into to various government services. The idea is that you already authenticated to the bank when getting the account, so they can be sure it is really sou when you log in - so why not make it possible for you to log in to other services as well if you want to ?

So we trust a bank more than the government that they won’t extend this to earn more money by disclosing more information? Bad idea. You need a neutral broker.