There is a sense in which aspects of reading level out for the majority of the population. That is, decoding and fluency skills essentially plateau at the late middle school/early high school level because they are easy enough cognitive tasks for the majority of the population to grasp (including some cognitively quite low-functioning individuals). (And one of the reasons derived grade equivalents are a problem on NRT.) Vocabulary and comprehension can continue to build beyond that level, of course.
This is my feeling about it as well. Fluency like arithmetic has a fixed level which we expect almost everyone to reach. So yes after a certain point the age at which reading starts loses significance. If however, early reading is just a sign of intelligence then obviously by the point that occurs the child will have moved onto other more significant achievements that will be noticeable.
