Presumably Mastodon is not on the under-16 ban for if someone can work out how to set up a Mastodon account, they also would easily be able to circumvent any security settings.

@davidallengreen a group of teenagers under 16 could set up their own mastodon server and NOT federate, just like anyone else.

Just them and their friends.

#fediverse

@knowprose @cstross @davidallengreen at that age, we had our own Fido-compatible network (an "FTN" — Fido Tech Network) where we were copying homework and planning parties, using the phone line.only for a few seconds every hour.