I would say that this show is for at least older teenagers. The cover blurp would have you think that the reason is violence. BTW, the main on-screen violence is in the first episode and the last two. I believe that the real reason is that it is ostensibly about a delusional, psychotic teenager whose delusions become real. In addition, other people also have delusions, and the delusions have delusions. The result is that in order to properly follow the story, you need to be able to follow what is real and what isn't. Otherwise you will end up being very confused.
The story opens with our hero, a teenage otaku, who is living in a metal shipping crate on the roof of a building. He has a sister who visits him periodically, but he spends most of his time with his delusional anime girl-friend. He periodically goes to school. If he isn't gaming, he is doing on-line chatting with his friends. As the story opens there have been two sets of murders. The pace takes off when someone sends him a pic of a horrific murder. The next day (at night), he witnesses that murder in progress. He sees a girl he doesn't know at the scene covered with blood, with the murder weapons in hand. He runs, and is caught in a surveillance camera, so now the police are searching for him.
The next day at school he finds that he is sitting next to the girl, who says that she has known him all his life. Other girls enter the story, one of which says that the hero himself sent the email with the pic of the murder, and others that say he should awaken. Then the girls start carrying swords. Then the EVIL organization enters the action. Shibuya will never be the same, since it gets pretty well destroyed.
The end result is a psycho-thriller, where you are kept wondering what is real and what isn't. If you liked the psycho-thriller Perfect Blue, I think you will like this one also.
The package comes with four disks, two DVDs and two Blu-Ray. I watched the Blu-Ray and the quality was very good. The main characters were hand drawn and the swords were CGI, which added to the impact of their supposing to be delusions made real. The extras were some trailers, text-less credits, and a song.