Honestly, I think the answer is subjective. Some kids can read "older" books, while others can't or don't even want to.

When I was really young, I was never interested in reading because it was boring. Then, when I was about 10, my older brother years older) gave me Have Spacesuit Will Travel to read, then Glory Road, and I turned from not reading at all to obsessively reading all of his books (he was clearly heavy into sci-fi and fantasy). I made it to Dune at the age of 11.

Personally, I don't see a problem with reading it that young. To this day, it's one of my favorite books. Of course, I read Friday and Time Enough for Love (also by Heinlein) when I was 12, and that might have been a bit young for those. I still loved them, though.

With WoT, I didn't read it until my husband insisted I read it, then plowed through nine books (all that were out at that point) in about two weeks. I gave TEotW to my niece around that time, and she was about 14. Her mom read it first and approved of her reading it and even gave it to my nephew, who was 12 at the time. My nephew hated it, but my niece liked it enough to read the first couple of books, then quit.