I've offered to my nine year old that she can read the Reckoners (though she hasn't yet) and she's read Skyward and Starsight and loved them. My personal metric was the Harry Potter books--book seven is real dark, with lots of death and so forth. She'd read that and handled it fine, and I'd say that a kid who can read and be okay with the later HP books will be fine with Sanderson YA content. The hobbit isn't nearly as dark, though, so it just depends on the kid.
(Edit: I said the Hobbit isn't as dark, but now I remember being real sad about Fili and Kili as a kid. Still don't think it's as violent? But I haven't done a side-by-side comparison.)
