Ursula Le Guin has a short article on why Fantasy is the literature with the widest age range of readers of the same works. I don't know, I've know some precocious kids working on some science fiction works that adults enjoy. Do 8-year olds only start with fantasy? She thinks science fiction begins with pre-teens.
The Harry Potter phenomenon, a fantasy aimed at sub-teenagers becoming a great best-seller among adults, confirmed that fantasy builds a two-way bridge across the generation gaps. Adults trying to explain their enthusiasm told me: "I haven't read anything like that since I was ten!" And I think this was simply true. Discouraged by critical prejudice, rigid segregation of books by age and genre, and unconscious maturismo, many people literally hadn't read any imaginative literature since childhood. Rapid, immense success made this book respectable, indeed obligatory, reading. So they read it, and rediscovered the pleasure of reading fantasy - which may be inferior only to the pleasure of rereading it.