User blog comment:FairyRaven/Fairy Tail Chapter 344: Wizards vs. Hunters, Review and Commentary/@comment-1359064-20130719155328/@comment-7418318-20130719174430

It can't be a copyright issue with One Piece for sure, because they didn't invent it (and didn't get sued over it either).

There was a Star Trek TNG episode that turned the captain and a few crew members into children and that was in the 80s, which means roughly 20 years before the first chapter of One Piece was released.

And I'm sure Gene Roddenberry (or whoever wrote that episode) wasn't the first to come up with that concept either, but it's the earliest example I know of without needing to check.