unfortunetly, you cant do a movie character exactly like the cartoon version, every live action movie has its suttle diffrences, look at the movie " Kick Ass " the main character Kick ass in the comics doesnt have a mouth opening for his mask, yet the movie version does.
Look at " the Incredible Hulk " Liz Tyler who plays Betty Ross, although her reprisal of the roll was superb and down too the very point of the character, she is missing one key factor, Betty ross in the comics is a blonde not a brunette, also Nick fury isnt black in the older comics, hes only black now because they needed more prominant black characters in comics, but in the original 1970's too 1990's hes white with brown and silver hair.
Im not mad, Im just shooting in the reality of hollywood and SPFX are expensive and some movie budgets dont have the funding for that kind of work, besides its cheaper too get a kid young enough too play the role then too higher and actress that requires cg affects too make look younger.
Since this movie is a trilogy and the first movie is based on Book 1, then obviously they need actors that can stand the test of time because they are going too age just like the actors did in the Harry Potter Flicks, so ang isnt going too look 12 forever, and obviously neither is Katara.
Im hopeful for the films since M.night is doing a style that isnt his own, we can only wait and see what happens