I don't know why the "What parent's need to know" shows Violence and Language on the same level. The language used isn't generally mature, just intelligent, and a bit... crazed... at times. The only curse word I can recall being used (in the actual show) was "Damn", used later in the show, and that's only a few times. In the second intro (which, unless you are watching the English Subtitled version, isn't very noticeable), the F-bomb is used. That being said, the Violence should be either a 3 or a 4, because a lot of people die, not in a particularly gore-y fashion, mind you. There are a few more negative messages you could get from the show, but watching it all the way through makes it more positive, though there is one confirmed suicide and another implied suicide, among a few other things. There really aren't any sexual references, and the clothes the females wear (except for Misa at times) aren't too skimpy, and the women aren't glorified for the men to like, in fact (and this is one of those things that sends a bad message), the main character Light doesn't really like any girl or boy throughout the show, but he does fake it from time to time, and he generally treats everyone except for his family (his dad is sometimes exempt) like they are tools to accomplish his goals, not people, women included. There is a small amount of drinking, I believe, as well as one character who is seen smoking for a few episodes. There is some slightly scary imagery, as well as some psychological mind games going on, but nothing that will scar your child for life or anything. I think 12+ would be fine, but you might think of waiting until 13, provided your child is allowed to watch more edgy stuff (and, let's face it, the majority of the kids at school do, too).
Note: I rate it as a 5 star because I personally love the anime. That isn't to suggest I think it is "the best!" thing to show your hypothetical privileged and sheltered 12 year old.