I read both The Hobbit and LOTR so young that I literally do not even remember reading them for the first time -- I don't know when I first picked up my dad's paperbacks, but I'd definitely read them all by kindergarten: my mom had to fight with the school administration to get special permission for me to go to the school library along with the first-graders. Apparently they didn't bother taking kindergarten kids because we weren't supposed to be reading well enough to go, and she had to make a huge fuss about how I was already reading high-school-level books on my own.

The Silmarillion I read for the first time when I was nine -- I can date that exactly because it was a Christmas present the year it came out, and I still have that much-battered inscribed copy on my massive shelf-o-Tolkien.