I might take the movie in. I often thought that Aang was portrayed *too* much like a carefree American kid.
But he was raised by monks, and he had recently (give or take a century. :-) )been told that he had an important job to do.
I suspect that a young boy raised in a life of great discipline would not act like the Aang in the cartoon.
That's a very good observation, and the Aang in the movie is more in line with that type of behavior, as opposed to the Aang in the animated series.