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Not sure if the Avalon books by Marion Zimmer Bradley are too complicated for that age, but I think for a girl they are more interesting as there is less sword fighting and barbarian hero worship.
Anne McCaffrey's Pern series has sex scenes far more suitable for younger readers-- it's there, sure, but it's all in these vague and euphemistic terms with no details provided, and there's always a scene cut. I think there's no sex at all in the harper/drummer/whatever series, and the euphemistic sex is in the series about the dragonweyrs or whatever. (It's been a while since I read them.) They're not great literature but they're really entertaining and a whole lot less pretentious than a certain other series mentioned in this post.
So that's my two cents on that issue.