Matthew is six (click on my name to see a picture). He thoroughly loved it. He's listened to the Hobbit and FOTR on audiocassette, the BBC radio version and was familiar with the story as well.
Prior to taking him to see the movie, I downloaded the moria scene and we watched it slowly together. When it happened he was familiar with it and it didn't frighten him that much. Really, there was not a whole lot of blood, just very fast violence that ended pretty quick.
I was describing to my husband last night the part of the ROTK where the orcs catapult human heads into the city during the seige of Gondor. I wondered aloud how they were going to manage that bit of cinematography!
Before I got to your post about taking your Jedi, I was going to make my comment of how I was horrified at how many young children were in the theater! Reading the books and/or listening to radio readings are one thing, seeing the extremely frightening images coupled with emotional music at high volume is quite another. I would
never imagine taking my five year old daughter to this movie as I can just imagine the nightmares she would have from the images seen.
I'm not saying you're a bad parent for taking your son, I just think there are few children who could see this movie without having nightmares from it.