According to the text, about what percent of the adult population either maintains a stable level of intellectual performance or increases their performance well into their 70s?

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The text notes that when we look at how individuals change as they grow older, we find that 45 to 60% of people maintain a stable level of overall intellectual performance-both fluid and crystallized-well into their 70s. An additional 10-15% show increases in performance into their mid-70s.

According to the Text, About What Percent of the Adult