Bella,
I'm not sure why publishers market to that age group, especially when a fair number of women read fantasy. However, that is the general market. It's roughly the same market that comics are marketed to. Historically, that's the fan base. Publishers aren't interested in demographics or genders. They are interested in sales. So because that's the group these things are often popular with, that's the group they market to. Most of the Forgotten Realms, Dragonlance, Ravenloft, Warcraft, etc novels are written with a particular formula in mind, and they are marketed to that age group.
Ursual LeGuin has written fantasies very different from the mold. I particularly liked Mercedes Lackey at one point. She has some great female characters, but I think I outgrew her work after a while. Marion Zimmer Bradley, C.J. Cherryh, Andre Norton and there are several younger authors who've followed in the same vein introduced "strong" female fiction. These are not the Tolkienesque clones however.