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This is a variant of Joseph Campbell's Hero's Journey. Adolescent characters are hardly new, Dorothy from Wizard of Oz, Jason of the Argonauts, Theseus (of minotaur slaying fame), and more recently...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/25195 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
This is a variant of Joseph Campbell's Hero's Journey. Adolescent characters are hardly new, Dorothy from _Wizard of Oz_, Jason of the Argonauts, Theseus (of minotaur slaying fame), and more recently the kids from Narnia, Garion from David Edding's _Belgariad_, arguably Frodo from LOTR, etc. These characters have the advantage of little to no backstory of merit, other than perhaps their parentage. So the audience gets their entire arc, as opposed to something like Odysseus, who had prior adventures before the Odyssey. The Farm Boy to King plot arc is pervasive and very compelling, even to adults. They serve as a readers entry into a fantasy world as well. Specifically marketed towards kids/teens, a protagonist of their age/experience allows the reader to more fully relate to the story, like having Jim Hawkins in _Treasure Island_, with every other character being an adult what is the appeal for kids? Perhaps super-hero fiction deviates from this more than other youth media, but even then there is Robin, Spider-man (the most commercially successful superhero by a mile), Jimmy Olsen, etc, all young heroes/sidekicks that serve as a stand-in for the youthful audience. And of course comics have been trying to kick the "it's all for kids" association for decades!