Seconding what aeh said, the less extreme scores (closest to the middle) will show the most difference between age and grade percentiles. I've seen it on my son's percentiles too.
I saw less of a gap between age and grade rank this year because they tested them earlier in the year, so that with a late December birthday he was almost average age to take the test. I wonder if this is also part of the reason why his scores were higher. Maybe he was hitting a bit of ceiling previously - I don't know at what ability level that becomes a concern with the cogat.
