I've said it before, and I'll say it again: The standard should be that devices ask whether the user is a minor during setup, and make that available as an is_minor boolean to all apps and websites. Children's devices are almost always set up by parents, and the setting can be protected by a parental PIN code. This method is effective while being completely private and local.

Though I can't take credit for the idea. It was proposed by the European Democratic Party.[0]

[0] https://democrats.eu/wp-content/uploads/2025/12/Protecting-C...

It's only effective if "Children's devices are almost always set up by parents", which is a big assumption. My parents were about as tech savvy as you could reasonably expect but I still got away with buying R-rated video games and such. Kids are persistent and the dangers aren't always obvious.