Excellent point, indigo.

But to the question of predictive correlations specifically for cognitive assessments: yes, those .8ish numbers have been found in other research as well, with other citations in the .7s. If you look at the Scottish Lothian Birth Cohort longitudinal studies, you'll see numbers in the .5 to .6 range (these are in comparison to old age, though).

My response to one of your posts on another thread may be relevant too.

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