Readers of this genre want to believe in the change. A character who is naive in chapter one and wise in chapter twenty, with nothing but time in between, feels fake. The growth has to come from choices, mistakes and consequences the reader has watched.
They also want specificity. A real kitchen, a real school bus, the particular smell of a particular summer, the song that was always on the radio. Universal feeling grows out of concrete detail, and a vague setting makes the whole book vaguer.
Voice carries enormous weight here, perhaps more than in any other genre. Whether the narrator is looking back from adulthood or living through it in real time, the voice must feel young enough to be convincing and clear enough to be readable. Retrospective narrators get a second job: they must know more than they once did without lecturing.
The ending should feel earned and slightly bittersweet. Growing up usually involves loss, so a perfectly happy ending often rings false. A hard-won understanding rings true. Many readers also want a mentor, a rival and at least one adult who fails the narrator, because those relationships mark the edges of growing up.