I dont think the book was origionally directed at 11 - 15 yr olds, the context is quite mature, and well written.. And the movies are rated M arnt they? Therefore people have acknowledged that it shouldnt be watched by people under 15..
We live in the age of Bratz dolls. Mature content is being targeted at pre-teens and early teens regularly. The novels are classified as "teen" or "young adult" paranormal romances- libraries are putting them in the section dedicated to 11-15 year olds (my mum works in a library, I've seen how it's classified).
Well written? It has almost no discernible plot, two-dimensional characters that undergo no character development, it is riddled with grammatical errors and defies the most fundamental, basic, primary-school rules of story-telling. That's not innovative; that's lazy. Just when you think there's going to be a climax.... Oh, wait, no, anti-climax it is. You cannot have an omniscient first-person narrator, either. She cheats at narration!
Defend the enjoyability of the series all you like- it's a matter of taste. How well it is written, and the psychological soundness of the content however? Those things are undeniable.