I thought about this while reading the excellent When You Reach Me, by Rebecca Stead. It's really middle grade, not YA as Miranda, the first person narrator, is eleven years old. I still enjoyed it immensely, because Stead is a good and clever writer. Part of her cleverness shows in the setting: New York City in 1979, an era that to today's American kids must seem as alien as a foreign country. Grownup readers, on the other hand, will identify with the memory of $20,000 being a huge game show prize. I can remember the year well, even if I know the city only slightly. And though I was a lot older than Miranda in 1979, I can remember being eleven well enough to know Stead has nailed the feeling of being that age. The story is told in Miranda's voice, and the voice is authentic as well as likable. The story also has spec fic underpinnings, although it's not obvious at first. When You Reach Me has been very well reviewed by librarians and teachers because it works well on many levels and has so many intrinsic elements— humor, mystery, friendship, moral behavior. Plus, it engages your sympathy for the characters and makes you care what happens to each of them.
Try doing all that in a book that can be read and enjoyed by everyone from age ten on up! So much for dumbed down.