I read this to mean that by fourth grade there was no advantage to the kids who learned to read at 2 over those who learned to read at 6.
I agree with some of the other posters that the problem here seems to be the failure to differentiate between gifted and normally developing kids. I've read somewhere (wish I could remember where) that the gap between gifted and normally developing kids only widens over time, and I've definitely seen this with my son. He read easy readers at 2 with no formal instruction and is now reading 5th and 6th grade material fluently with comprehension at age 4. As a result of his early reading and advanced comprehension, he entered kindergarten with an expansive knowledge base that also gives him a huge advantage in things like science and social studies. His teacher has commented that he makes great contributions to class discussions and that his classmates learn from him. Unless you took all books away from him for a few years, I honestly don't see how his classmates could catch up at this point, given his huge head start. For example, I doubt any of the other kids in his kindergarten class (who are all older than him) have read Greek myths, while he's already committed dozens of Greek heroes and stories to memory and refers to them in unrelated contexts. By the time his classmates get around to the same stories years from now, DS4 will presumably have long since moved on to other things.
Maybe normally developing kids who read early don't comprehend or retain the information they're reading the way gifted kids do? Or maybe they can read, but don't have the same passion for the knowledge imparted by reading that many gifted kids show? Otherwise, I don't see how there can't be some lasting advantage.
