I watched this anime last summer and I really enjoyed it and I am very glad that I watched it now. I like the suspense factor in the anime as well as the plots in certain episodes. There are some scenes in the episodes I enjoy, and there are others I don't like but overall it is a great anime. One of my favourite anime along side others like Ai Yori Aoshi and Planetes.
As for Age Rating I would give it 15+, this seems like a fair age rating. There is a bit of violence in it but anyone around this age or older would be mature enough to watch it.