Mine aren't quite there yet -- at 8 and 10, their lexiles are in the 1000s and 1200s respectively, but their stamina doesn't hold when it comes to doing more than a few dozen pages at that level.
But even as a college-educated adult with a high IQ, I find that it takes building up to get to the point of being able to keep focus on a long book with complex sentence structure, arcane vocabulary, and intricate plot. So I'd just kind of keep that in mind, that the skill of sticking with and unpacking a complicated piece is different from the skill of reading a few sentences of a certain complexity. The thing that has only become apparent to me in the past few years is that this skill is definitely something one can build progressively! It's like weight-lifting or distance running: you can't jump straight in to a marathon, there's some intermediate steps -- even if you're in shape to start with.
Happily, I recently rediscovered my bookmark of this amazing thread about progressively building reading levels...you might find it interesting, too.
http://forums.welltrainedmind.com/t...gh-progressively-more-challenging-works/
