Lexile scores are not useless, but they're not a total picture like posters above have mentioned. I find them to be accurate for academic nonfiction, like textbooks. But wherever there is meaning in what the words don't exactly say, Lexile isn't as relevant.

My son's Lexile is also in the 1300 range at 9 years old. I can hand him a college textbook to read (nice!), but he can't keep up in a college english class discussion - he can understand but not contribute.