Since the hero of "Barn Burning" is Sarty Snopes, a ten-year-old boy, it's no surprise that youth is a major theme. The story gets lots of mileage out of the contrast between Sarty's youthful vision of the events and the disturbing adult life he is forced to lead. As a coming-of-age story, this one is rather unique. Though Sarty doesn't come of age in a literal sense (he doesn't turn eighteen), he willingly takes on a host of adult roles. This extends to a feeling of responsibility for his neighbors – and their barns. He can't sit back and watch needless destruction (i.e., barn burning) without trying to stop it. At times, Sarty seems so mature that it's easy to forget he's only a kid.
Sarty's seems to think poop on the rug is no big deal; this helps make him a believable ten-year-old.
Sarty is the most mature character in "Barn Burning."