Last year, my DS's lexile level was given as 1100-1300. He was 8 and in 3rd grade. The teacher insisted that his independent reading books all had to be in his lexile level. It wasn't until I asked her to print me a list of recommendations that she went "Oh, wow, uh yeah..." because she realized that there was almost nothing in that range that would appeal to an 8 year old and most of what did was totally inappropriate for his maturity level.

I find that it's the variance in material, the exposure to different types of writing and a true love of reading that matters far more than lexile level. My son devoured the Percy Jackson series- which brought a passion for mythology that has lasted more than a year.