No. If you are operating a p2p video chat site that is accessed by minors and you have no way for users to block or report content, then you are indeed going to face issues.

If a user is downloading a Linux image via bit torrent, or using WebRTC data channels to synchronise many clients using a p2p mechanism, what is the risk that needs age restricting?

This is what I meant, the OP is factually incorrect that just adding P2P technology to a service means it must age gate.

And if so, is BitTorrent also completely illegal now, even if there's no piracy? IPFS? I2P? Tor?