I can't remember how old I was when I read it... I just looked it up, and found this: "It’s part of the Faber Children’s Classics list and is often recommended for middle-grade readers, typically between 8 to 12 years old."
Although it is scary, I think it's interesting what Catherine Storr said about her children's books (from her Wikipedia biog):
Storr's books often involve confronting fears, even in the lighthearted Polly stories, and she was aware that she wrote frightening stories. On the subject, she writes: "We should show them that evil is something they already know about or half know. It's not something right outside themselves and this immediately puts it, not only into their comprehension, but it also gives them a degree of power".
I actually think this is what makes the book so brilliant -- as children, we do have nightmares and feel terror, but what do we make of that and how do we process it?
Thanks for reminding me of Alan Garner! The Weirdstone of Brisingamen was one of my favourites, so spooky and wonderful.