Pointed noted, Val. The data set that I linked is the WJIII standardization pool, which is indeed representative of the US population. Nothing is said about the IQs of the test subjects because that is what was being obtained during standardization. It is as generalizable to +3 SD as any other large-scale population sample is, including those we use to derive IQs. (After all, it literally is the data used to derive IQs.) Which is to say, better than nothing.

The other point to consider about that data is that it is not longitudinal, so it tells us mainly about the distribution of cognitive and academic skills in people of different ages, not directly about how cognition changes over the lifespan.

Thanks, indigo, for improving my verbal expression!

...pronounced like the long vowel and first letter of the alphabet...