Some of the best fantasies ever written are books written for children. An adult reader who is serious about fantasy literature will need to check the children's shelves at their library or bookstore.
Need convincing? Well, here is a very brief list of some of my favorites children's fantasy, linked to articles that should convince anyone that these books may be taken quite seriously by adults.
Also, keep in mind that not everyone agrees on what is children's fantasy and what is adult fantasy. The Encyclopedia of Fantasy, in the children's literature entry, notes that after the success of Tolkien created an interest in adult fantasy, many children's fantasies were marketed as adult fantasies. For example, A Wizard of Earthsea by Ursuka K. LeGuin was published as adult fantasy in the US but as children's fantasy in the UK.
The point of this discussion is that I would hate for anyone to miss out on some of the best books ever written because somebody labeled them "young adult" or "Ages 8 and up".