I'm not going to rewrite what others have written about regarding the violence and cursing, etc. Most have done an excellent job and they are pretty spot on. While there was a bit of cursing, it wasn't simply gratuitous for the sake of just saying it like many other movies are. Additionally, I was not cringing every time I heard it. Usually, there was enough action, or plot rather, going on that the words were not quite the focus.
As far as the violence - to be honest, I was expecting more based on the reviews that I saw here. From my recollection, there are only about 5 or 6 scenes that were fairly graphic, but nothing too off-putting. It's hard to say sometimes, but again I do not think it was gratuitous and fit with the movie, if that makes any sense. Most were over just as quickly as they began.
Finally, we are a Christian family and the plot of the movie was interesting and different and I was entertained the whole time; however, it wasn't really the best movie I have seen. There are some various Christian messages, but I am not sure it was really intended to be a full out Christian message. The story is more about Eli and his "mission" than it is the Bible or God. God does seem to play an important role and there are a few times that some good messages are shared by Eli. If I had to give one theme that stood out among the rest it would certainly be faith - Eli exemplifies faith in spite of adversity.
The main review of this movie above does bring up an interesting point that I thought myself - it is hard to tell if the director and writer of the movie was simply developing the "bad" character of the villain or rather presenting perhaps their own view of religion - but there are some hints that organized religion is simply a way to gain power.
As far as kids - we watched this with our 12 year old and I must say that as parents, we must know our own children and their maturity level. Age is just a number, and I know many 17 (and plenty that are older) who are very immature and know others that are younger that are more mature. I would have never let our 11 year old watch this movie - he has a problem with watching violence and doesn't know how to handle it. Our 12 year old though is very mature and quite a strong Christian. He can separate reality from the movies and understands what is okay and what is not okay. He, like myself, focuses more on the story than the cursing and the violence.
On that note, parents only you know your kids (at least, I hope that you do) and you should not let ratings alone determine what you let your children watch and not watch.