Obviously, Jamie is maturing in any case, but I think that providing richer narrative experiences helps people to form more complex ideas. It is (perhaps) easier to see this happening in children than in adults, because the ideas they are coming to understand are ones we already know, so we can see the improvement more easily.
Bérubé points out that when Jamie was a baby he was told not to expect him to learn to read, while a generation earlier he would have been told not to expect him to talk. A strong illustration of how high expectation and rich experiences can broaden horizons - for anyone.
It's a nice read, and I think you might like it if you are interested in children's literature or the relationship between story and mind.