We just took ds1 + 4 friends (9s and 10s) to see Eragon, and they all loved it.
Younger children would be either too scared by it, or not have the concentration to follow it and be bored.
It is quite long (just over 2 hrs), and is quite slow in places. The plot is not always easy to follow, and it helped that several of the party had read the book, so whispered explanations to the others!
It is quite scary in places - big battles, 'monsters' - like LoTR but not as bad, plus sad bits. We took ds2 (7) and I think he was really a bit too young: he seemed OK, but I'm withholding judgement til I've seen whether he has nightmares tonight!