I'm more familiar with Australian legislation than others, but here at least a home server would definitely not require age verification. Kids are free to make group chats with their friends in a bunch of services.

The spirit of the law is definitely not against chatting with friends, but it is against the idea of connecting minors with strangers, so while federation is generally not codified (or, IMO, understood well by legislators) and you're probably not going to be bothered by authorities about it, I reckon sooner or later the law will come for federated networks.

(Since we all seem fine just taking some uncertified random third party's word for it that their AI face recognition definitely didn't see a thumb with a face drawn on it, maybe it'd be adequate for Matrix.org to add an "18+ user" flag to the protocol and call it a day?)