>The social skills learned in such an artificial environment are highly unlikely to transfer to the real world.

That's an odd argument, do you think there is little socializing benefit from any kind of schooling then?

A 12 month range of age is not unusual for a circle of peers/friends throughout life, especially if you normalise for age.

A narrow age band means that children are likely to be at similar stages of emotional & mental development as their classmates, so they can learn from each other. It's important not to forget that children are children and not adults. School shouldn't necessarily directly model the rest of their lives.