I know you used the word maybe, but I personally think the tight age banding of middle class life has nothing to do with the improved outcomes for kids. Or at least, there's no reason to think it does. Much more likely is superior education and not having to deal with adult stresses at a young age (though I think even the latter can be a good thing, when it occurs in the context of a supportive environment and an education that encourages kids to analyze such experiences).