memes

Another comment here gives an example of how a 6th grade reading comprehension test could be formulated. Essentially, it's about how complex sentences you can parse, and how large your "context window" is while reading.

Imagine a small child just learning to read. They struggle with every word, so if a sentence grows more complex than "The dog is brown.", they simply can't get to the end of the sentence while still remembering what the start was about. This also applies at a higher level: Keeping track of a complex "scene" which describes a setting while also describing dialogue between characters and inner dialogue in parallel requires more cognitive effort than the simpler "scenes" in children's books. A higher reading level means you spend less cognitive effort reading and understanding the words and sentences, so you have more cognitive capacity in reserve to actually understand the full picture.