Actually... The age changing fits in perfectly into what we have going so far. Though temporary AUs sound completely amazing too.

Although, one thing is that it would likely need to take multiple probably Hat Kid's whole stash Time Pieces for a game wide effect like that. A single Time Piece could certainly affect multiple characters, but it would be much slower and probably deliberate, depending on how and who's using it. It could be a very drastic change, like somebody tried to alter something that happened in Far Shore itself. Tried to write another character out of existence and now has to rewritten by being rescued out of a Time Rift or something. Or maybe Ammy is suddenly an adult for a couple days until it wears off, for something potentially lighter.