I read a Wrinkle in Time when I was in 5th or 6th grade and it BLEW.MY.MIND. I think that is definitely age appropriate for good readers.
The sequel (A Wind in the Door) is a bit more advanced. I don't think it is suitable for younger than about 7th grade, if memory serves. The science in the book confused me until I read it again in high school.
Swiftly Tilting Planet is advanced. High school for sure IMO.
Wrinkle in Time was wunderbar, but I seem to remember it as really really scary. One of the few books that ever terrified me. Beautiful, Beautiful writing though. Marvelous characters. May be Definitely a favorite.
Wrinkle in Time scared the crap out of me too.
Mercedes Lackey! Anne McCaffrey! C'mon, all those fantasy ones that you loooooooved at that age?
Or, Tamora Pierce (definitely not very scary)
Yeah, Wrinkle is scary, although Abby just finished reading it and loved it. Swiftly Tilting Planet is my favorite of the original trilogy.
Oh, did someone mention Robin McKinley? Beauty, Blue Sword, Hero and the Crown, Spindle's End -- almost all her stuff should be suitable for that age.
What about Wrinkle in Time scared you all? I don't remember being scared through it.
But...I was reading mysteries and the like from a young age.
For me, it was the inhuman villains. All those children, bouncing balls at the same time, skipping rope to the same rhythm. It creeped me out very effectively.
Yeah, I don't remember the L'Engle books being scary either. Thinking about it, I'm pretty sure I was always bothered more by suspenseful scenes than by bad stuff actually happening. So, like, a character lost in a creepy swamp and hearing weird noises for a chapter was terrifying. A character actually being eaten by wolves? Not a problem.
I read all the Black Stallion books around that age.