It solves the problem of your kid borrowing a phone from another kid at school.

Does it though? Unless all countries unify their laws regarding this matter it will fail in the same way that blacklist filtering does.

Also I'm not convinced that borrowing a device presents a new or different failure mode. Children could always obtain physical contraband from their friends so nothing has changed here.

no it doesn't

also doesn't need to IMHO

it solves the problem of it being too trivial for a 12 year old to access content which at best is quite problematic and at worst outright traumatizing

as in, the same reason we have laws that a clerk glances at the age on you id if you look young and buy alcohol but your parent are still allowed to let you drink with them if they think its right (or what a 16+ movie with them etc. etc.)

this is also why it really shouldn't be anything much more fancy then parent controls checking min age of content locally / indication of age for feed fetching. Everything else is disproportional (unrelated from all the other issue it might have).