If we're only talking about mental age (i.e., we're leaving fine & gross motor skills out of it), I'm not sure I agree.
Take reading. A typical 3 year old will be learning pre-reading skills and a typical preschool classroom will be focused on basic skills like enjoying books and learning the alphabet song. A typical 6 year old will be learning to sound out simple words and a typical 1st grade classroom will be focused on decoding skills. A typical 9 year old will already be a fluent reader, and a typical 4th grade classroom will be much more focused on content. They are three distinct stages, and I am equally horrified at the thought of that ND 6 year old struggling to learn in a classroom of kids who are fluent readers as I am at the thought of a ND 3 year old struggling to learn in a classroom of kids who are already sounding out words. (Of course, the 3 year old is much more likely to be running around like a whirlwind than attempting to learn, but then we're getting into emotional/social age.
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I'm not sure about this either - but while I agree with what you are saying in terms of classroom activities and reading levels, that's not really what the ratio I.Q.s measure, is it? I think you are talking more about acquired skills. The different age kids will have different readiness levels, but that's partially experiential? If you took all three ages and exposed them to a mental activity none of them had any prior experience with (yes, I am stretching), I think the gap would be larger between the six year-old and the three year-old. My gut (and it's only my gut, I can't back it up) is that a six year-old has a lot more in common with a nine year-old than a three year-old.
Again, I'm not convinced about any of this and I'm not trying to be argumentative. I think the premise bothers me because it's basically saying "A 130 I.Q. child has as much in common with an average child as an average child has in common with a retarded child." And since it's a safe assumption that most people's kids are pretty close to average.....well, it's just not something I would recommend saying, especially if it's not entirely accurate (maybe it is, I'm not convinced yet). I'm imagining the parent of a child with a 130 I.Q. going into the child's school (which might be in a highly educated community where the average I.Q. is 115, not 100) and making this statement. Don't think it would go over well.
I realize that it has illustrative power, I just think there are more accurate (and more sensitive) ways to get the point across.
