You play young characters so that you can grow in their world with them and gain and understanding and viewpoint about their worlds at about a normal age that they would.

Also, a young character is pretty much a naive innocence. The only kind there is for a seed that will someday blossom into a hero. It is common for the protagonist to be 'not smart' and in need of guidence in a romance and tragity. Thus, a child is common cast.

Bipper