I think this is a reasonable question. However, I would argue that it still should be.
Firstly, I should say that reading scores aren't typically measured by grade levels for this type of study. That's just a colloquialism we use to make it comprehensible to the average person. The PIAAC for example uses a numerical score that translates to "levels of competency". [1]
Still, I think it's still a valuable way to express the idea. There exist levels beyond the sixth. Even if most folks don't attain those higher levels anymore we do need some way to refer to them and the sixth grade is when a high school bound adult should have attained that level in order to keep up with later coursework.
Firstly, I should say that reading scores aren't typically measured by grade levels for this type of study. That's just a colloquialism we use to make it comprehensible to the average person. The PIAAC for example uses a numerical score that translates to "levels of competency". [1]
Still, I think it's still a valuable way to express the idea. There exist levels beyond the sixth. Even if most folks don't attain those higher levels anymore we do need some way to refer to them and the sixth grade is when a high school bound adult should have attained that level in order to keep up with later coursework.
[1] https://nces.ed.gov/surveys/piaac/skillsmap/