FWIW, the way I've always understood the "third grade" bar is that it doesn't mean all kids are thought to suddenly be the same iq/ability... but instead means that around 3rd grade is the time you start to see most kids fall into where they will ultimately be based on ability - so for instance, a child who started reading late but is going to be a great ahead-of-grade-level student in reading will be at that level by 3rd grade - but way back in kindergarten might not have been reading yet. Another child, otoh, who started reading at three might not end up in the "top" reading group in third grade because they were an early reader, but weren't ultimately a hugely-over-the-top-in-ability reader. That's a bit of an exagerated example to make a point - but fwiw, that type of "evening out" absolutely did happen in my children's classes.
My kids are early bloomers, so I've been keeping an eye on this. We have definitely known some kids who seemed bright but who were not doing nearly what DD was doing in K who have "bloomed" as the children have aged. So, I do see some evening out--though it is worth saying that what I see now is a bunch of kids who seemed bright but there wasn't really concrete evidence for it now showing concrete proof, but some kids are still ahead. One thing that might muddle things and lead to this observation is that at a certain age, kids don't want to read much past their maturity level. This has been discussed here--what 9yo really wants to read for pleasuure at 1200 Lexile? So bright kids and HG+ kids may all be reading Harry Potter in 2nd or 3rd grade.
I don't know of any children who read at 3 and then evened out. But I don't hang with pushy parents.
