There are a lot of different charts that give you an idea of the expected Lexile ranges for each grade. One such chart:
http://www.neisd.net/read/tech_tools/database_training/lexile_chart.pdf They aren't all *exactly* the same, but this should give you a general range.
Assuming that a 9 y/o would, on average, be a 4th grader, you can assume that the average (reading right at grade level) 9 y/o would have a lexile around 700-800 or so.
Dd#1, who has always been a strong reader, had a lexile of 1250ish at 9 and 1468 at 10. Both placed her at or above the 99th percentile for grade level. She was in 4th grade at 9 and 6th grade at 10 (having skipped 5th).
