Indigo makes some excellent points. Interest level, lexile level, and emotional maturity do not always align. For example, "Of Mice and Men" is a classic high school text, on the reading list for a wide range of educational philosophies. Yet its lexile level is only 630, which is often described as on the lower end of the middle grades reading range (grade 3-5 or so). Some of the chapters, taken by themselves, don't even make it into the grade three range. But I don't think anyone is claiming that Steinbeck is too easy for sixth graders. Romeo and Juliet, on the other hand, is also a typical tenth- or eleventh-grade literature selection. And it's above a 1300 lexile. These are both -- nearly unanimously -- considered classic high school readings.
And fwiw, the Redwall fantasy series, popular among many middle grade readers, goes up above 1000L, with scores inching up more-or-less the further you progress in the series.
