

Extant information is safer than just uploading an id (which is what the laws want). No one needs to know your birthdate or identity if your account is 18 years old. No one needs to know your birthdate or identity if you have a cc on file (you gotta be an adult to have a credit card and therefore to have credit card on file).
This is the best possible solution, where there isn’t a requirement to upload a scan of a government issued id.

It’s under settings > Apple account (the bubble with your name if you’re signed in) > age range for apps.
The first time you click it you’re asked to go ahead or update your birthday in the account first.
Inside you can pick to always, never or ask first before sharing your age range with some app and it says you can see what apps have asked for your age range.