Fantasy particularly is notable as the only form of literature that consistently blurs the boundaries between adult fiction and children's fiction.
Lewis Carroll's books Alice's Adventures in Wonderland and Through the Looking Glass, for instance, have defied description since they first were published over 100 years ago. The books unquestionably were written for a child--the original Alice, who was the daughter of a friend of Carroll's--but I would wager that they are just as often read by adults looking for an escape back to a simpler time.
The books written by Theodor Seuss Geisel under the pseudonym "Dr. Seuss," arguably the most popular children's books of all time, can be described as nothing other than fantasy. Their wild and colorful worlds, inhabited by wild and colorful creatures, are the setting for tales that hold merit both for their inventiveness and for the real emotional and social values they espouse.
A more modern example is J. K. Rowling's Harry Potter series, which--literary acclaim or no--have caused something of a revolution in the publishing world. While these again are aimed at a youth/young-adult market, they have become a "trendy" read for socialites and hipsters alike of all ages.