The Immaturity Stone

The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Immaturity Stone. It worked just like the normal stone, except that every time the stone was used, the one making the wish would regress in age by one year. Reality would adjust itself so that everyone (except the wisher) would think that he or she was supposed to be whatever age he or she had become by using this stone.