Aang and Toph are the only ones who are 12, though, at least at the series start -- Kat and Azula are both 14, Sokka is 15, Yue was 16 (and that was marriage-age in her tribe) and Zuko 16 -- but something close to a year has passed in show continuity so that can all be tweaked up. (Three years haven't passed in the show -- but there is an episode where Iroh mentions that three years have passed since the then-14-year-old Zuko was scarred and banished, which might be where folks get that idea.)

As for Jet, I like him because I find the character interesting, not because he's likeable, if that makes any sense -- to my eyes they're setting him up to *look* like the cool bad-boy-rebel sort, but then they undermine the facade of coolness by showing how manipulative and ultimately nihilistic he is. He's a rebel without a cause, and I like how he serves in the story to disillusion Kat and the others about falling for slick flattery and a cool image.

A couple of fic recs -- may_flyer's Progression is a dark, unflinching look at the development of the nihilistic lashing-out-at-everything Jet, and ladylisse's Long Long Time Ago is a near-future AU where Aang has failed, and the new Avatar has been born into the Foggy Swamp Water Tribe -- gorgeous writing and characterization, and it makes the swampbillies into a beautiful, complex culture instead of the one-note joke seen in the show.