I'm hesitant to place an age on it because all kids are different. Heck, even my almost 7-year old has responded more favorably to movies where I thought the characters might be creepier.
That said, he 1) started crying out of sadness about 15 minutes before the movie ended because an innocent creature ultimately died (yes, the dog died at the beginning, but we knew he came back to life and all was fine or him...) and 2) that night he said he couldn't get the images of the dead animals out of his head. This was not him just wanting to stay up later. I laid with him and could see his eyes wide open, being very alert and tense.
I said there is a positive message because part of the story is based on doing things with love and for the right reasons, or you might get a bad result. Not sure kids will catch that on their own, but the science teacher mentions it and it seems to apply to the rest of the events (although, interestingly enough, I don't think that philosophy is real in the situation used). Also, there could be a message that being the introvert (which is not viewed as being as good as extroverts in our society) is not a bad thing. That introverts can be successful, kind, etc. Again, it's not obvious and would warrant some discussion.
My husband, who jokes that I always want teddy bears and rainbows (not true, LOL), even said it was a little more graphic than he had expected. Not showing blood or amputated body parts or anything, but the acts of violence were creepier.