My almost-7 daughter and 9 year old son were riveted to this one, very excited while watching it, but afterwards said they didn't want to watch it again. However, the conversations about good and evil, materialism, and the way various characters were portrayed went on for a while afterward, they were very stirred up by it.
I would not let the younger one watch it alone, this was one she needed a little reassurance for the first time due to the suspense, violence, monsters. She said she liked it a lot, but that it was a little too scary. So I would say she is the absolute bottom end for this one (7 in two months).
I think the violence would be too much for a younger child. Also I think younger children would just be bored, have a hard time tracking the story which really jumps around a lot, and with the lack of concrete normal time/space reality. A 9+ kid who loves history, has been reading about knights and Ancient Greece, would probably be the best young audience.
The Satanic character is linked to the game shows and mindless materialistic greed of the protagonist's parents.
Throughout the movie the boy is making strong heartfelt moral protests against the materialism and evil behavior of the bandits. Killing and war are mostly shown as futile and evil (if sometimes necessary, when Agamemnon kills a Minotaur figure). The bandits attempt to defeat the Satanic character with soldiers out of history and he easily turns their attacks fatally against them.
One of many fine surreal touches - the evil being's castle is subtly shown to be built out of giant Lego bricks.
The Supreme Being is shown as a very civilized calm bureaucratic character in a gray three piece suit, a little like a wise cranky strict fatherly headmaster, who hates a mess and is in total control.
There is a lot of grim grimy humor and a lot of cartoonish death, though a loved character is killed (temporarily) at the very end. No swearing. There is some wine drinking and cigar smoking at various points (by the adult bandits, who are clearly not role models).