"Aang kind of was the previous Avatar. He spent a century frozen in an iceberg, so a new one couldn't be born but he also couldn't do anything about the war. At the time he entered the iceberg he was twelve, so he hadn't been announced as the Avatar before that - when they can swing it they don't tell anybody including us until we're sixteen; Aang knew and some monks knew but that was it. The previous previous avatar, Roku, is said to have tried to do some things to calm down the guy who started the war in the first place, but died and the war kicked off."