Belgariad around 14. And Wheel of Time I began at 15. Though WOT I would have begun sooner had I discovered it sooner.

LOTR is relatively straight forward but both WOT and Dune do better if the person is somewhat familiar with East/South Asian history and philosophies/religions. It's not that they can't be enjoyed, but it's enjoyed on more levels if one has that background.

I think it depends on what year we are discussing and how good the library's science fiction/fantasy section were. And if you discovered these things on your own or had an older sibling who liked these things.