When they become an adult you can use most of the items. If it's a V2 you can't use the ball as an adult. With v3 you can... Young Mametchi can turn into Mametchi, otherwise what's the point in calling it Young Mametchi?! If you have a V3 I'll tell you one thing: Mametchi has been redrawn, and Obotchi can turn into Mametchi.