I first read this book when I was in Fourth Grade, and just finished re-reading it. Of course, when I first read it, I didn't fully understand what was happening, nor did I completely understand what Annabel was going through.
Now, having read it a second time, and being more mature, say that this book is good for 12 year olds, if they are mature. I, for one, am a mature thirteen year old so it wasn't a big deal for me. But parents should decide based on how mature their child is before allowing them to read it.