My Daughter is now 13 and read both The Enemy & The Dead. She absolutely adores them and were the first books that she ever really got in to. Since then she has read the Gone series- She liked the fact that it was just a world of kids that was left- No Grown ups. But, saying that, they could be slightly gory and upsetting for a child of a nervous disposition. Hope it helps x
My 12 year old says this (and The Dead) is her favourite book. My 9 year old has tried it but finds it a bit scary. I think it depends on the child and his or her exposure to other horror-type stories.