The hero of a fantasy novel tends to be young1, and tends to come of age in a fantasy novel.
BUT
That doesn't mean that fantasy novels tend to be bildungsroman -- a formation novel, or less literally, a coming-of-age novel. Such as To Kill A Mockingbird, or Jane Eyre, or Anne of Green Gables.
Not that they are non-existent. But the genres don't pull in the same direction without work.
Plots of a bildungsroman are, on the whole, of a much looser weave that, say, an epic fantasy. Or, really, any other kind of fiction.
Which is why plot skeletons tend to fall down with particular spectacle when dealing with them2. Analyses of Jane Eyre that pare off everything except the romance and feign the rest doesn't happen, or explicitly declared the other parts superfluous. Or treating To Kill A Mockingbird as having the plot of the trial, instead of its being a subplot providing Scout with more examples for her learning how to properly treat other people.
In an epic fantasy, the conflict of the story is between the hero and the Dark Lord or other evil force. This requires tying down of the action and events. Too many happenstances lead to muttering and the exasperated quoting of Aristotle:
Of simple Plots and actions the episodic are the worst. I call a Plot episodic when there is neither probability nor necessity in the sequence of episodes.
In a bildungsroman, the conflict is between the young character's ignorance and ill-formed character and what he needs to learn. The happenstances can fit because they are unified by having happened to this character and having been formative influences on him -- particularly if they show a clear pattern of growth, so there is probability in the sequence.
The events do, however, need a certain intrinsic probability to them, which is why bildungsroman are easier when mundane. An out-of-the-blue announcement that the crotchety great-aunt is visiting is not out of the ordinary. We all know of the existence of great-aunts, and that some are crotchety. The announcement that the Golden Stag, which grants wishes, appeared in the Golden Wood needs a little more foreshadowing. And probably subtle stuff, so that it seems like part of the local color.
This is why most fantasy bildungsroman works are cozy. Low-key fantasy settings where the hero's school or the heroine's apprenticeship are not drowned out by the circumstances in which they occur. Or where the Golden Stag can grant minor wishes.
A bildungsroman -- to change the metaphor -- is a meandering stream weaving its way through the park, while an epic fantasy is water cascading down a gorge in a score of waterfalls. Once you have established the meandering as the way the story goes, you do not want to interrupt with a sharply focused, one-conflict gorge -- especially not if you go back to the meandering afterward.
I once read a novel where the writer couldn't make up her mind which she was writing and so jerked me back and forth between incidents in the life of the heroine working as mercenary soldier and an external conflict driven story. It doesn't work like that; in a bildungsroman, the main character's development is always central.
An external conflict and a bildungsroman can be mixed, but it takes skill. In that case, the external conflict must be the driving force of the character development, and vice versa. It's not just a matter of interchanging them.
The character change can be demoted to subplot, but then, of course, the story must be ruled by the external conflict, with the subplot working around it.
Likewise, the bildungsroman can take place during dramatic historical times, but the emphasis must be on the character's development. Consequently, it, for instance, features a boy of a boatman family as they ship food in to allow a siege of the Dark Lord's castle to continue, and he assumes greater responsibilities in the course of the tale.
A bildungsroman's one real plotting advantage is the planting of guns on the mantelpiece according to Chekhov's famous rule. Because the incidents are important as they happen to the character, you already have their first purpose, and can produce them again, later, for other purposes.
Its one real plotting disadvantage is that you still need a climax to it. A crescendo. Something to sum up the conflict and bring it to a conclusion. And since it's internal, you somehow need the setting to marshal a suitable demonstration of what he has learned. Meanders do not lend themselves to dramatic conflicts. I notice that in such works, there tends to be more focus and cause-and-effect in the incidents as the work goes on, so it does not seem disparate if it -- well, a gorge would overdo it, but often it ends in a waterfall to bring the tale to a conclusion.
Which is a challenge in any genre.
The Youth of Heroes
Not just in YA, but also in adult fantasy, the hero of a fantasy novel is prone to be on the young side. Just barely an adult, or on the brink of being one.
A Brief Note On Story Structure
There are plot skeletons out there, recommending the form they give to the writer, especially the new writer. There are -- quite a few actually.
Oh, yes, this reminds me of something I was complaining about the other day. I'll have to cudgel my brains and remember it.