You don’t need age verification if you run it for your family and know everyone’s age.

You could run your own family forum just fine. The problems start when you want to federate. Let me be crass to make the point. Say someone posts child porn and that gets federated to your instance. You think you can just declare that someone else's problem to avoid legal complications?

The way I expect this would work, is that instances would become responsible for who they federate with. If an instance allows your family instance to federate, they would allow your users to indirectly use their instance. We'll have to wait what lawmakers or courts do, as you say. But I think, federation would only be by manual approval after some sort of check for compliance, or maybe even a legal contract similar to how it goes in GDPR. Actually, such GDPR contracts might be required anyway, but who cares.