I will differ from other reviewers in saying that this show is not really about zombies. Walking Dead is an excellent character drama that explores themes of true human evil. The concerns that parents should consider when researching this show cannot be confined to the little review clickboxes about violence, sex, and so on; the show goes darker and deeper than that.
Topics that have come up in this show include, but are not limited to: rape, molestation of minors, spouse abuse, cannibalism, infanticide, complete mental breakdown, euthanasia, torture, betrayal, tyranny, cult psychology, execution of a child, execution of the ill, and more.
So when deciding whether to allow your tween or young teen to watch this show, the question is much deeper than whether they will find the zombie gore troubling or not. Ask yourself whether your child is ready to contemplate the deeper themes of evil in the show.