I think I actually started reading the Robots and Foundation series the year after I read those books - first or second grade, probably.
Hence the problem with longevity and marketability for "young adult" lines of books. Really avid readers often tend to leap from picture books and/or comics straight to quite adult novels.
Kids who are so-called reluctant readers may or may not catch the independent reading bug depending on the "must know" quality of the content of the YA novels they happen upon. Then they, too, may leap for adult novels. Or simply stop reading altogether.