That doesn't work, unless the system knows everyone's family relationships.

Not guesses. Not is told about and takes on trust. Knows.

There's nothing to stop a kid creating a fake adult account and using it as an adult, perhaps creating their own kid account for "official" use.

Ultimately this is an unsolvable problem without a single source of truth for verified ID and user age.

The only responsible way to do that is to create a global "ID escrow" agency, where ID details are private and aren't available to governments or corporations without a court order, but the agency can provide basic age checks and other privacy services of a limited nature.

Good luck with that idea in this culture.

Meanwhile we have the opposite - real ID is known to governments and corporations, personal habits and beliefs of all kinds can be tracked, there is zero expectation of privacy, and kids still aren't protected.