The California law requires everyone to show their age, not just kids.

No it doesn't, at all. In fact it specifically says it only applies in the case where it's a parent setting up a device for a child.

You can read the actual law, it's short.

(a) (1) “Account holder” means an individual who is at least 18 years of age or a parent or legal guardian of a user who is under 18 years of age in the state.

(2) “Account holder” does not include a parent of an emancipated minor or a parent or legal guardian who is not associated with a user’s device.

(d) “Child” means a natural person who is under 18 years of age.

(i) “User” means a child that is the primary user of the device.

(1) Provide an accessible interface at account setup that requires an account holder to indicate the birth date, age, or both, of the user of that device for the purpose of providing a signal regarding the user’s age bracket to applications available in a covered application store.