A Wrinkle in Time is actually considered Middle Grade and it's too bad we insist on these labels. A good book is a good book no matter how old the protagonists are or what the reading level is judged to be. Some of the best writing is currently happening in the 8- to 12-year-old range. (If you liked A Wrinkle in Time, you'll most likely enjoy When You Reach Me by Rebecca Stead. It won the Newbery Medal a few years ago.) Sadly, it's mostly only parents of said age group and librarians who know this.
Then there are the few cross-age authors with a huge following whose "kids" books sometimes get shelved the adult ones (Neil Gaiman's The Graveyard Book, for instance) at least by some independent bookstores. To them I say, hooray!