Well i suppose once children have learnt to read, there isn't much further they can go. So I don't think it's so much about slowing down or leveling out, with regards to reading. Of couse some children will never have been taught properly, there will some variety in adults who have.
The first few years at primary school seem to be about writing, reading, basic arithmatic. That's what my sons teachers seem to be obsessed with Once childern have (meant to) learnt these basics of communication, maybe education turns the emphasis to what, and how are they being used? There's many aspects to learning to read and use written information/
So could give the impression that some children level out, some slow down, some late developers blah blah blah
My summer born, year 2 boy, was pretty good with words at 3. Nothing amazing but he could sight read words, picked up letter sounds without much imput. He just seemed to know! Then Reception and Year One he wasn't that interested and was a nightmare. First term of year 2 and he starts trying to tackle chapter books. Next term and suddenly he can pretty much read most words, he doesn't have to think much anymore. Not so much levelling out, or catching up, it was a bumpy ride
I was an early reader and loved reading, but one of my brothers really struggled, not only with reading but many aspects of school. At home he was a bright little boy but the school had a very different opinion. By the time he was 15 he was avidly working his way through my bookshelves. Interested in, and happily tackling books that covered all sorts. I don't think I've turned out better for being an early reader, he hasn't suffered by being a late one.
Some early readers will go on to be very "academic", some later readers may go on to be very "acadmeic".