DH and I went to see it yesterday and we both loved the movie. He is a
major Batman fan (he actually owns the domain
darknight.co.uk 
)
He loved the movie and thought it was an excellent example of Christian morals in many instances. I do agree with him, but the violence for me was a bit much. HOWEVER - I have to say that it was essential to the development of the Joker charicter and is needed to put his insanity in a context that shows you just how evil he is, and how that evil manifests itself in his actions - why he does what he does - not just violence for violence sake.
I think that when it comes to the proper age it’s going to depend on the child. I’m sure there are some 17, 25, or even 60 year olds who are not up to it, and some 13 year olds who will be just fine. Do not make this determination based on the rating, but on what you know about your own child and how they may be able to handle the content.
Oh - by the way - there is not one single instance of foul language, nudity, sex, or blood/gore in this move. Violence yes, but no blood. Only one
very brief scene of a couple smoothing their clothing when they are “caught” - but you don’t ever see them doing anything, and both are fully clothed, just rumpled.
~Liza