I think it all depends on how mature your kid is, and how much they read/have read in the past. I've read the first one of these, and I would say that when I was 11.-12 I would have loved them. I might not have gotten all the jokes, but I would have loved them. I can also say that the woman that worked in the library at the time when I was 12 would not have approved at all! Mean, nosy old bird, she actually called my parents when I checked out Brahm Strokers Dracula at age 14... It was too mature for me she thought!
You as a parent is (or is it are thebest judge? Damn the english language!) the best judge of your child's level of maturity, I very rarely follow the age suggestions on books, unless it says 15+ or adults or something like that.
I read the book, and then make my mind up, and lets face it, when it comes to young adults books they don't take that much time to read, and you can form an opinion pretty fast.