Cohort bias ^{Reference Lezak, Howesen and Bigler4}

It is tempting to say that one could answer these questions by testing a large group of 25-year-olds and a large group of 80-year-olds without any known cognitive impairment and attributing any differences in neuropsychological test results to the effects of age. However, if we were to do this in 2023, we must remember that persons born in 1998 and persons born in 1943 may have had, on average, very different life experiences. Access to education, nutrition, exposures to various toxins, technologies, social stressors, and many other things may have changed and may have contributed to any differences we observe. Thus, it would be difficult to confidently attribute any observed differences solely to the effect of aging.

Recruitment bias ^{Reference Minder, Muller and Gillmann5,Reference Ford, Howerton and Lai6}

In recruiting participants for clinical studies, we are always dependent on who is willing and able to participate. For example, persons who are ill may be unable or unwilling to participate as may those in difficult social or financial situations. This may mean that we tend to end up testing only the healthiest and wealthiest individuals, potentially biasing our results toward testing subjects prone to do well on cognitive testing and tending to lower our chances of detecting cognitive changes with aging.

Misclassification bias ^{Reference Lezak, Howesen and Bigler4,Reference Ross, Abbott and Petrovitch7}

In studying normal aging, we may inadvertently include subjects with neurodegenerative diseases or other neurological or systemic conditions that have not been diagnosed and thus the group we think represents normal aging is in fact contaminated with subjects who are not normally aging. Although conditions affecting cognition can occur at any age, they are more common as people age. A group of people who start off healthy will be prone to developing illnesses with age that may affect cognition. This would tend to make an older group perform worse on average and lead us to overestimate the effects of age in a healthy population. Any real-life population at any particular age will include both healthy individuals as well as those with diseases affecting their cognition.

Practice effect ^{Reference Abner, Dennis and Mathews8,Reference Salthouse9}

We have seen above in number 1 that testing different cohorts at the same time may be misleading so it may seem that the correct way to do things is to test a group of subjects at one age and then retest them periodically. For example, we might test a large group of 25-year-olds and then retest them annually until death in an attempt to uncover the effects of aging. Unfortunately, we may see practice effects where persons tested on the same or similar tests repeatedly may improve or not decline as much as they should, leading us to underdetect decline with aging. Studies of this type also introduce huge logistical barriers in finding and following up the same people over such a long periods of time.

Survival or attrition bias ^{Reference Van Beijsterveldt, van Boxter and Bosma10}

If we follow a large group of subjects over years, not only does this become an expensive and difficult study to sustain but we will also lose subjects as the years go by to death, disease, physical frailty making participation difficult, and so on. If we are only testing the healthiest subjects, this may tend to underestimate changes with age.

Neuropsychological testing is not real life ^{Reference Salthouse11}

Most studies in this area are based on standardized neuropsychological tests administered in controlled environments. Subjects are focussed on the tests and distractions are minimized. If we see declines on testing results with age, this may or may not translate into real-life problems as life experience and accumulated learning may mitigate adverse effects. It is difficult to measure the effects of accumulated “wisdom,” which may improve an individual’s abilities in certain tasks. Driving is one example of a real-life skill that has been studied. ^{Reference Doroudgar, Chuang and Perry12,Reference Lloyd, Cormack and Blais13} Studies in this area consistently show that car crashes are more common in teenagers than in any other age cohort. ^{Reference Tefft14} This rate goes down with age and then rises slightly after about the age of 70. However, people over 80 years of age still have a lower rate of crashes than do teenagers. ^{Reference Tefft14} Of course, there are many confounding factors. People over 80 may not drive as much as teenagers do, for example. However, I bring this up to show that we are not very good at measuring the effects of experience and accumulated knowledge on ability.

Salthouse ^{Reference Salthouse11} explores the paradox of why healthy elderly individuals do so much better in real life than they do in the laboratory. For example, while laboratory studies of analytical reasoning show declines with age, the ability to complete crossword puzzles continues to improve throughout life. Although age-related declines in job performance have been described in air traffic controllers ^{Reference Trites and Cobb15} and athletes, ^{Reference Baker, Deakin and Horton16} most studies have not shown a close relationship between age and job performance ^{Reference Scahill, Frost and Jenkins1–Reference Rhodes22} across a wide variety of occupations. It has been suggested that although age may affect the limits of cognitive abilities on neuropsychological testing, our occupations do not routinely test the limits of these abilities day to day. ^{Reference Salthouse11}

Effects of sensory decline and physical changes

Senses become less acute with age. For example, hearing acuity typically begins to decline after the age of about 30. ^{Reference Goman and Lin23} Most persons over 80 have significant hearing loss. Testing needs to account for the fact that older subjects may have more trouble perceiving stimuli. Physical limitations of aging may also make response to testing more difficult.

Fewer studies of the very old

Very old people are fewer in number in the population. Fewer of them are still healthy enough to participate in neuropsychological research and more of them have acquired neurological disease that affects their performance so that we are no longer necessarily studying normal aging. Many studies have examined normal cognitive functions up to the age of 80, but the studies beyond that age become few and far between.

Processing speed

How quickly tasks can be performed begins to decline in the twenties and does so steadily throughout life. ^{Reference Salthouse26–Reference Carlson, Hasher and Zacks28} Tasks as simple as reaction time steadily decline throughout adulthood. ^{Reference Gottsdanker29} This is considered a fluid ability. This decline can affect one’s ability to do other cognitive tasks and lead to false conclusions about the effect of age on other cognitive abilities. For example, verbal fluency is often measured by the number of words an individual can say from a specific category within a minute. ^{Reference Borkowski, Benton and Spreen30} Slower processing will affect the ability to perform this task despite preservation of language skills.

Attention

Performance on complex attentional tasks declines with age. ^{Reference Salthouse, Fristoe and Lineweaver27,Reference Carlson, Hasher and Zacks28} Simple attention tests such as digit span (a subject is asked to listen to a series of numbers and then recite them back) are not significantly affected by age at least up to the age of about 80 after which there is a slight decline. ^{Reference Lezak, Howesen and Bigler4} Selective attention is an ability to focus on specific stimuli while ignoring other irrelevant information in the environment. This skill is needed to carry out a conversation in a busy environment or to drive a car. Divided attention is the ability to focus on multiple tasks at the same time. Following a specific walking path while simultaneously reciting the alphabet backwards would be an example. Carrying out a conversation while driving might be another. Working memory, the ability to briefly hold information in memory while at the same time manipulating the information, is considered a complex attentional task and also declines with age. ^{Reference Salthouse, Mitchell and Skovronek31} A real-life example might be calculating a restaurant tip in one’s head. Working memory, selective and divided attention steadily decline throughout adult life, beginning in the early twenties.

Memory

Some aspects of memory remain stable during normal aging while others decline. Immediate, or sensory, memory remains stable. ^{Reference Lezak, Howesen and Bigler4} This is often classed as a simple attentional task and would include tasks such as reciting back a list of numbers as mentioned above.

Long-term memory of autobiographical or historical events remains essentially stable throughout life. ^{Reference Ronnlund, Nyberg and Backman32} However, attribution memory or source memory does decline with age. This facet of memory refers to knowing where one heard a piece of information – “Did my spouse tell me that, or was it a co-worker, or did I read it in the paper?” This has been shown to steadily decline between the ages of 21 and 80. ^{Reference Meusel, Grady and Ebert33,Reference Casino, Torres-Trejo and Estrada-Manilla34}

Semantic memory, or memory for facts about the world (e.g. Canadian confederation happened in 1867, many leaves change color in the autumn), tends to be relatively preserved and, in some studies, even to improve into old age.

New learning may be measured by free recall, i.e. asking a subject to recall a list of words previously given. Free recall declines with age, declining relatively slowly until the age of about 60 and then declining a little more quickly. ^{Reference Price, Said and Haaland35,Reference Cargin, MaruffP and etal36} Recognition memory (yes, that item was in the list you showed me earlier) tends to be better preserved with age. ^{Reference Fraundorf, Hourihan and Peters37} As we age, retention of newly learned information tends to be preserved, but retrieval of that information may require cuing or a recognition task to recall it as recognition memory is better preserved than free recall. ^{Reference Price, Said and Haaland35–Reference Fraundorf, Hourihan and Peters37}

Procedural memory, i.e. memory of how to carry out physical tasks such as riding a bike or typing, is well preserved with age. ^{Reference Churchill, Stanis and Press38}

Prospective memory or remembering to perform a task at a certain time (e.g. putting out the garbage on Monday evening) tends to decline with age. ^{Reference Cargin, MaruffP and etal36} Subjects in their twenties perform better than those in their sixties who perform better than those in their eighties.

Executive Functioning

This refers to the complex set of abilities that allow us to plan, focus our attention, manipulate information in working memory, inhibit inappropriate responses, shift mental set, exhibit cognitive flexibility, and multitask successfully, all skills that are important in a successful work and home life. With age we see a decline in the efficiency of carrying out novel tasks, as well as in those where we need to distinguish relevant from irrelevant information or stimuli, and in those where some responses must be inhibited but not others. These abilities typically begin to decline from the sixties onward. ^{Reference Lezak, Howesen and Bigler4,Reference Salthouse26,Reference Allain, Nicoleau and Pinon39–Reference Treitz, Heyder and Daum43} There are few investigations of specific executive abilities. One study suggests that inhibition and updating may be more impaired in older subjects while flexibility is not. ^{Reference Etienne, Marin-Lamellet and Laurent42} Another suggests that inhibition and division of attention decline after age 60. ^{Reference Treitz, Heyder and Daum43} Crawford and Channon found that, while older subjects did more poorly on standardized neuropsychological tests of executive functioning, they performed as well as younger subjects in real-life situations. ^{Reference Crawford and Channon44}

Language

Most language abilities tend to be well preserved with normal aging and some, such as vocabulary, may improve throughout the lifespan. ^{Reference Allain, Nicoleau and Pinon39,Reference Folia, Liampas and Ntanasi45–Reference Verhaeghen47} As noted above, verbal fluency may decline with age, but this does not necessarily reflect an underlying decline in language abilities and may reflect a decline in speed of processing. Similarly, auditory comprehension in noisy environments may decline, but this is generally related to age-related hearing loss. ^{Reference Verhaeghen47}

The tip-of-the-tongue phenomenon is a universally experienced inability to produce a word in a timely fashion accompanied by a feeling of partial knowledge of the word and that word retrieval is imminent. Its frequency has been reported to increase with age and may underlie poorer performance on naming tasks despite increasing vocabulary with age. ^{Reference Facal-Mayo, Juncos-Rabadán, Alvarez, Pereiro-Rozas and Díaz-Fernández48,Reference Facal, Juncos-Rabadan and Rodriguez49} Some evidence suggests that age-related declines in processing speed and working memory may be contributors. ^{Reference Facal, Juncos-Rabadan and Rodriguez49}

Visuospatial functioning

These are variably affected by aging. Performance on Block Design, the ability to use colored blocks to replicate a presented pattern, declines steadily from about age 30–80. ^{Reference Klencklen, Despres and Dufour50,Reference Joung, Yi and Ahn51} Recognition of objects, shapes, and signs is well preserved into old age. ^{Reference Lezak, Howesen and Bigler4} Copying a simple figure does not seem to be affected by age but copying a complex figure such as the Rey-Osterrieth figure may. ^{Reference Gallagher and Burke52} Judgment of spatial orientation of lines or objects becomes steadily more difficult with age. ^{Reference Joung, Yi and Ahn51}