I'm not saying you specifically have a 6th grade reading level, but if the average is lower, it's going to change the understandings of the majority. If your parents (just as an example, not saying they have this reading level) have a 6th grade reading level, there's going to be a difference in your vocab growing up than someone with parents with a college degree. Geographical location, education, whether or not you're an English Language Learner, friends, peers all affect your vocab and understanding of the language. There's nothing wrong with this, which again, is why I was trying to explain why "lame" could be viewed as ableist.

My mistake, it was 7th-8th grade. source

I linked the fallacy fallacy because you didn't cite any sources, only gave your personal experience. This is why I responded with my personal experience. The first definition in the dictionary is the ableist use. It was not an old ass person, not sure why you made that assumption