In the intelligence field, there is a distinction between:<ul><li> "fluid" intelligence (indexed by tests of abstract reasoning and pattern detection);</li><li> "crystallized" intelligence (indexed by measures of vocabulary and general knowledge).</li></ul>

Fluid vs. Crystallized intelligence

There's no age at which humans are performing at peak on all cognitive tasks - our various cognitive functions peak at different times and past a certain age it might make more sense to view adult intelligence not through the lens of youthful general processing speed and reasoning, but through the lens of expertise, wisdom, and purpose.

The peak of our cognitive functions

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When does cognitive functioning peak? As we get older, we certainly feel as though our intelligence is rapidly declining. (Well, at least I do!) However, the nitty gritty research on the topic suggests some really interesting nuance. As a <a href="http://journals.sagepub.com/doi/pdf/10.1177/0956797614567339?casa_token=L29u1CmuY6gAAAAA:HwbJVxHMdjeQxoe1A1UcjnE-sTEy1YhbSyhlQic62On1RpUMpAd0JKWh7H2IvB3tj5dmN8nw2g4x">recent paper</a> notes, "Not only is there no age at which humans are performing at peak on all cognitive tasks, there may not be an age at which humans perform at peak on most cognitive tasks."
In one <a href="https://journals.sagepub.com/doi/pdf/10.1177/0956797614567339?casa_token=L29u1CmuY6gAAAAA:HwbJVxHMdjeQxoe1A1UcjnE-sTEy1YhbSyhlQic62On1RpUMpAd0JKWh7H2IvB3tj5dmN8nw2g4x">large series of studies</a>, Joshua Hartshorne and Laura Germine presented evidence from 48, 537 people from standardized IQ and memory tests. The results revealed that processing speed and short-term memory for family pictures and stories peak and begin to decline around high school graduation; some visual-spatial and abstract reasoning abilities plateau in early adulthood, beginning to decline in the 30s; and still other cognitive functions such as vocabulary and general information do not peak until people reach their 40s or later.
The Dark Matter of Intelligence The picture gets even more complicated, however, once we take into account the " <a href="https://academic.oup.com/psychsocgerontology/article/55/2/P69/578710">dark matter</a>" of intelligence. As Phillip Ackerman points out, should we really be judging adult intelligence by the same standard we judge childhood intelligence? At what point does the cognitive potential of youth morph into the specialized expertise of adulthood?
In the intelligence field, there is a distinction between "fluid" intelligence (indexed by tests of abstract reasoning and pattern detection) and "crystallized" intelligence (indexed by measures of vocabulary and general knowledge). But domain-specific expertise-- the dark matter of intelligence-- is not identical to either fluid or crystallized intelligence. Most IQ tests, which were only every designed for testing schoolchildren, don't include the rich depth of knowledge we acquire only after extensive immersion in a field. Sure, measured by the standards of youth, middle-aged adults might not be as intelligent as young adults, on average. But perhaps once dark matter is taken into account, middle-aged adults are up to par.
To dive deeper into this question, Phillip Ackerman administered a wide variety of domain-specific knowledge tests to 288 educated adults between the ages of 21 and 62 years of age. Domains included Art, Music, World Literature, Biology, Physics, Psychology, Technology, Law, Astronomy, and Electronics. Ackerman found that in general, middle-aged adults are more knowledgeable in many domains compared with younger adults. As for the implications of this finding, I love this quote from the paper:
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"[M]any intellectually demanding tasks in the real world cannot be accomplished without a vast repertoire of declarative knowledge and procedural skills. The brightest (in terms of IQ) novice would not be expected to fare well when performing cardiovascular surgery in comparison to the middle-aged expert, just as the best entering college student cannot be expected to deliver a flawless doctoral thesis defense, in comparison to the same student after several years of academic study and empirical research experience. In this view, knowledge does not compensate for a declining adult intelligence; it is intelligence!"
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There was an important exception to Ackerman's finding, however. All three science-related tests (Chemistry, Physics, and Biology) were negatively associated with age. Tellingly, these three tests were most strongly correlated with fluid intelligence. This might explain why <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.391.5108&amp;rep=rep1&amp;type=pdf">scientific genius tends to peak earl</a> y.
Nevertheless, on the whole these results should be considered good news for older adults. Unless you're trying to win the Nobel prize in physics at a very old age, there are a lot of domains of knowledge that you can continue to learn in throughout your life. What's more, Ackerman found that certain measures of personality, such as intellectual curiosity, were related to domain-specific knowledge above and beyond the effects of standard measures of intelligence.
And even if you do want to maintain your fluid intelligence as long as possible, there is recent research suggesting that <a href="https://www.sciencedirect.com/science/article/pii/S1064748119300223">having a greater purpose in life can help protect again cognitive decline among older adults</a>. Giyeon Kim and colleagues combined seven items looking at various aspects of purpose, including plans for the future, importance of daily activities, dedication to ensure plans made are actualized in the future, a good sense of what one wishes to accomplish in life, whether one has accomplished all one wishes to accomplish in life, whether one cares about the future, and whether one has a sense of direction and purpose in one's life. They found that after adjusting for covariates, purpose in life acted as a protective factor against cognitive decline.* The researchers argue that purpose in life could be used as a treatment technique for cognitive decline in clinical settings.
Their research adds to a growing literature showing the <a href="https://journals.sagepub.com/doi/pdf/10.1037/a0017152">many benefits of maintaining a purpose in life for health and well-being</a>. Greater purpose in life has been linked to <a href="http://www.ahajournals.org/doi/abs/10.1161/circ.131.suppl_1.52">reduced all-cause mortality and cardiovascular problems</a>, <a href="https://journals.sagepub.com/doi/abs/10.1177/0956797614531799">increased longevity</a>, <a href="https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2648692">maintenance of general physical functioning</a>, <a href="https://www.sciencedirect.com/science/article/pii/S0022399913000391">reduced risk of stroke</a>, and <a href="https://link.springer.com/article/10.1007/s10865-015-9635-4">reduced incidence of sleep disturbances</a>. One longitudinal study over a 10-year period found that increased meaning in life was associated with lower allostatic load (the <a href="https://en.wikipedia.org/wiki/Allostatic_load">"wear and tear on the body"</a>). This is important considering that allostatic load has also been <a href="https://www.sciencedirect.com/science/article/pii/S0022399915005504">positively linked with increased risk of diseases, mortality, and cognitive decline</a>.
The good news for older adults is that not only can we continue to acquire domain-specific knowledge into older age, but <a href="http://jamanetwork.com/journals/jamapsychiatry/articlepdf/210648/yoa90079_304_310.pdf">purpose in life is also modifiable</a>. It seems that the question "When does intelligence peak?" is actually a rather meaningless question. Not only do our various cognitive functions peak at different times, but past a certain age it might make more sense to view adult intelligence not through the lens of youthful general processing speed and reasoning, but through the lens of expertise, wisdom, and purpose.
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* Interestingly, the results were particularly pronounced among older people and those who are black, and they did not find any effect based on sex.
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When does cognitive functioning peak? As we get older, we certainly feel as though our intelligence is rapidly declining. (Well, at least I do!) However, the nitty gritty research on the topic suggests some really interesting nuance.

When Does Intelligence Peak?

Fluid vs. Crystallized intelligence

The peak of our cognitive functions

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