MINOR SPOILERS!
Written for warning more sensitive viewers ;)
When I started watching this anime, I thought that I was gonna have a nice time but I warn you that this anime will take unpredictable turns in the story, touching some sensitive (or not) topics. When I started to watch the last episode, I was so scared that I looked up the synopsis for it (mind you, I was frightened by some previous episodes).
I recommend not watching it as a 13 year old but rather a 15 year old because it is much more violent than other 13+ rated anime. (Of course I'm not saying you can't watch it as a 13 year old, but it can get pretty dark).
I wish the anime would have stayed the plot it was the first few episodes because I enjoyed it very much.