It's an interesting legal question, but I would imagine for a federated service, the burden of proof should be on the individual's home server for age verificat

It's an interesting legal question, but I would imagine for a federated service, the burden of proof should be on the individual's home server for age verification. That's where the user account is, after all. Matrix is basically labeled "adults only" everywhere, so restricting certain servers/rooms due to possible innocent eyes is likely out of scope.