"Ender's Game" is the latest to put this idea in our heads. The adaptation of Orson Scott Card's 1985 sci-fi novel focuses on a young boy believed to be the best hope in saving the world from aliens. Based on how these sorts of stories go, we can assume he's successful.
The "child hero" concept is ancient, filling literature, particularly in books intended for young readers. And the same is true for family films where kids are depended on to save the day, whether they're trained and equipped for such a mission or accidental leaders, or a "chosen one." Below are a number of examples where underage heroes thwart a great evil that threatens us all.