There is no direct correspondence of a specific Lexile measure to a specific grade level. Within any classroom or grade, there will be a range of readers and a range of reading materials. For example, in a fifth-grade classroom there will be some readers who are ahead of the typical reader (about 250L above) and some readers who are behind the typical reader (about 250L below). To say that some books are "just right" for fifth graders assumes that all fifth graders are reading at about the same level. The Lexile Framework for Reading is intended to match readers with texts at whatever level each individual reader is reading.
With that said, we have a more detailed explanation and a chart available that shows Lexile ranges from actual test scores across the nation in each grade. Please keep in mind, the "Reader Measures" column of this chart is not to be taken as recommended ranges. This is simply where young readers are reading. And know that students scored above and below these ranges as well -- the ranges in the table are the middle 50% of students in each grade.