A writing student asked me if it's possible to write a publishable story about a character at a young age, for young readers, and then write about the same character at an older phase of life for older readers. Several years ago, I would have said probably not, but by now I've seen a few examples--and of course, now self-publishing is a more viable option than it used to be.
I've heard it argued that the Harry Potter books advance from middle-grade through young-adult. Author Brent Hartinger has taken his YA character, Russel Middlebrook (of Geography Club and its three sequels) into some new-adult books featuring the character in early adulthood (The Thing I Didn't Know I Didn't Know
and its sequels). Hartinger refers to the new-adult phase of the
character's life as the "Futon Years." (For other characters, this phase
of life might be referred to as the Dorm Years, the Studio Apartment
Years, the Living with Roommates Years, or the Sleeping on Someone's
Recently, thanks to a post on the Read is the New Black site, I was reminded of my affection for Marilyn Sachs's books, and I discovered she has a sequel to an old favorite of mine, A Pocket Full of Seeds.
That book took its main character from early childhood through the age
of thirteen, and the sequel takes her from age thirteen to seventeen.
Even though the books are billed as being for the same age reader
(grades 5 through 8), I suspect that the sequel, Lost in America, would appeal to somewhat older readers. I plan to check it out.
the point is that the rigidity of expectations about audience and
branding, and how older people won't read books for younger people and
so on, is fading. You may know of even more examples than the ones I've
been able to find. Even though it still might be unusual to take a
character into different audience age ranges, it's not unthinkable. And
the conventions that traditional publishing houses and booksellers still
follow don't have to apply to anyone who self-publishes.