When I started The Wicked Series, I went to Gregory MacGuire's official board, and was shocked to see how many kids were 8-12 when they started the series. 12 years old, I think is fine for a parent to allow their child, who they know can handle it, but, no matter how mature my child was for his/her age, I don't think I would allow reading the Series before age 12. For a more general age, I'd say not appropriate for under 14, without Parental permission
I read The Martian Chronicles
and The Silmarillion
before I was eight. I read Huck Finn
when I was eight; I remember that my mom had to give permission for me to check out Huck Finn
from the library. Kids can handle a lot more than people realize.