A‌ll of us age differently, and the extent to which our cognitive abilities fade varies considerably. Some of us will develop dementia, while others will retain our capacities into our 90s and beyond. What changes in the ageing brain? Why do individual differences exist? And what can we do to keep our minds sharp as we age? 

How cognition changes as we age

Not all aspects of our cognition deteriorates as we age. The most notable change may be cognitive slowing, with one study showing that a 20-year-old is 75% faster than a 75-year-old at substituting symbols for numbers. With regards to language, our vocabulary and verbal skills remain largely unchanged, but our ability to find a particular word can decline. Concerning memories, those we formed many years ago remain quite stable, but forming new memories or recalling things from the previous day can be difficult, and our working memory is particularly affected. Finally, although our ability to maintain attention on a single task is largely unchanged, we struggle to multi-task or switch attention. 

These changes reveal a pattern: although our general knowledge and ‘crystallized intelligence’ is mostly unaffected, our fluid intelligence—the ability to think on the fly and solve new problems—suffers as we grow older.

Dementia, mild cognitive impairment, and normal cognitive decline

Cognitive abilities in old age form a continuum, with normal cognitive decline at one end and dementia at the other; in the middle is mild cognitive impairment (MCI). Separating the three is not straightforward.

In dementia, the features of normal cognitive decline, as described above, become amplified. Rather than simply forgetting that a meeting was on, someone with dementia might not remember the meeting was ever planned. Dementia can also produce personality changes, confusion, problems navigating familiar environments, and difficulty performing everyday tasks. The biological markers of dementia include an accumulation of protein clumps in the brain, such as beta amyloid and tau in the case of Alzheimer’s disease. While these deposits may signal a neurodegenerative disorder, they can also be found—less frequently—in cognitively normal people. Risk factors include diabetes, high blood pressure, high cholesterol, lack of physical exercise, and a lack of mental and social stimulation.

Mild cognitive impairment exists in somewhat of a grey zone between normal cognitive decline and dementia, to which it is often, but not always, a precursor. The symptoms are similar to those for dementia—forgetfulness, impulsivity, irritability and difficulty maintaining a train of thought—except that they are not serious enough to interfere with a person’s independent function. The difference from normal cognitive decline lies in the severity of the symptoms, as assessed by a clinician. Risk factors for MCI mirror those for dementia. 

People who didn’t smoke, were physically fit, bilingual, and had more education, showed less cognitive decline than their peers

Individual differences in cognitive decline

Why do some people show significant mental decline while others seem unaffected? We don’t know the answer to this, and the question is more vexed than you might imagine. Problems arise because pathological features like beta amyloid and tau protein deposits are also found in healthy brains, and because the risk factors for dementia, MCI and normal cognitive decline are very similar.

While we don’t know what causes ageing-related cognitive decline, we do know what correlates with below-average cognitive performance in old age. This is not the same as cognitive decline, however, since poor performance in old age could be a continuation of poor cognition throughout life. To measure decline, individuals must be tracked over time, which is done far less often than simply measuring cognitive function in the elderly.

One study that did track a large number of people was the Scottish Mental Surveys, which found that the best predictor of mental performance in 77-year-olds was their performance as 11-year-olds. After controlling for this factor (which contributed ~50% of all variability), the researchers were able to show that people who didn’t smoke, were physically fit, bilingual, and had more education, showed less cognitive decline than their peers.

To the best of our knowledge, maintaining physical health and mental engagement seem like the most promising ways to limit cognitive decline.

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