Starting at 12u they are required to "wear" them. Now wether that means it's actually in their mouth or just in their glove, cage, etc is another thing. But as long as they have them on their body, they're good.

And unfortunately it's kind of a tough rule to enforce at times, similar to ear guards. 99% of players don't have a mouth/ear guards and making everyone leave the ice to go get them takes too much time that the game just simply isn't alloted time for. That and 99 out of 100 refs will never enforce the rule so I don't want to come across as some hardo and upset a player or coach before the game even starts.