Many researchers have reported that, after about age 20, scores on many subtests on intelligence tests decline, while scores on other subtests rise or stay the same after that age, and do not decline until much later. Subtests that fit the former pattern are typically measures of fluid intelligence, that is, the dimension of intelligence that reflects problem solving ability without the application of outside information, while those subtests with the latter trajectory are generally measures of crystalized intelligence, which is the ability to solve problems that require outside information Here are some pertinent quotes from the literature:
[F]or each variable [i.e., subtest] there is an increase until about the late teens or early 20s, followed either by a decrease for the process variables [similar to fluid intelligence], or by a period of stability and then a decline at around age 50 or 60 for product variables [similar to crystalized intelligence]. (Salthouse, 2010, p. 27)
Scores for fluid intelligence (e.g., short-term memory) peak early in adulthood, whereas measures of crystalized intelligence (e.g., vocabulary) peak in middle age… (Hartshorne & Germine, 2015, p. 433)
The growth of gf [i.e., fluid intelligence] is steady but relatively rapid and reaches a maximum in the late teens or early twenties, after which it shows a gradual decline…Scores highly loaded in gc [i.e., crystalized intelligence] on the other hand, show a more gradual increase from infancy to maturity, and…decline does not set in until relatively late in life, usually not until about 60 to 70 years of age. (Jensen, 1980, p. 235).
The truth, however, is that intelligence actually grows into at least middle adulthood (45-60), irrespective of whether it is measured using subtests categorized as being related to fluid or crystalized intelligence.
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“Bottom line: The article argues that if you account for practice and accumulated knowledge—not just raw speed—human cognitive performance often crests in the 50s–60s, not the 20s, with longitudinal data (e.g., PIAAC-L) providing the strongest support.”
The thing that I notice most is slower processing speed. Even though I feel I am at my peak in crossword-solving skills, I notice they are taking me a little longer to solve. My percentage of completions is at an all-time high. I have been doing the NYT Wednesday through Sunday crosswords for decades.
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