I find that statement odd, seeing as how the brain actually DEGENERATES as you become older. Perhaps the degeneration of the brain occurs in such a way that it appears to be a "leveling out" of one's functional abilities? Or perhaps the degeneration of one's functional abilities causes one to become less adept at the use of one's preferred functions, thus forcing the individual to use their weaker functions in order to compensate for the loss.

It should be noted though that visuo-spatial abilities decline the greatest with age, thus I find it questionable that functions associated with this ability, such as introverted sensing, whose proficiency in this skill I am still uncertain of, would have any noticeable growth in functioning at all.