A recent RGL evaluation of the NYT estimated the RGL at grade level 13. I'll point you at the study if you are interested. They market software to do RGL estimation these days.
If that's the case, then I stand corrected. When digging that fact out of my memory bank I may have misunderstood "13th grade" to mean "13 years old" (which is about the 8th grade). Thanks for correcting me on that.