Avatar: The Last Airbender's four-episode finale starts with a beach party. Sokka cracks jokes as he scrambles across a crumbling airship. The last spoken line is a blind joke. It is clear to me, in a way that it wasn't when I first watched it, that these characters are young teens. Young teens dealing with genocidal dictatorships, Orwellian city-states and the general mayhem of war, absolutely, but their age lends the whole affair a constant, underlying levity. The adults that exist are kept at arm's length from the action—present, but unmistakably marked as “grown-ups,” and thus distant. Youth, and all its connotations of hope and humor, are the engine of the show.
Legend of Korra, in contrast, is downright grim.