I don't know literature in this area, but I would assume that, yes, reading does even out at some point. What I mean is that at some point most people will read at 3rd grade level, or 8th grade level. It's just like all people sooner or later will learn multiplications. But I think the point of differentiation is that, while others learn to reach a 3rd grade reading level or math level, the kids who are already there should be learning something that they don't already know how to do. I just think the argument that "they will level out in a particular area" is irrelevant.
I'm very curious at your thinking here, as it may help unravel the myth. I would use "even out" or "level out" to mean at the same age kids all have the same ability. You seem to be using the concept differently.
I've also seen distinctions here where some people reference "reading" as just processing the words on the page and knowing the meaning of the sentences. Whereas as a category, I think there are different degrees of understanding and depth. With the same word knowledge and reading speed, one kid may read The Lord of the Flies and see it as an adventure story; another might read it and see it as a reflection on society and the behavior of mobs and norms, etc.
Similarly, two kids may read the same vocabulary list and both get 100% on the test. One will mostly recognize the words when the encounter them again, the other will see them as valuable resources and fluently include them in conversation and writing going forward.
