Many of us are familiar with the idea that as we get older, we become less mentally agile; but is this something that can be measured – does our IQ decrease with age?

If it does, at what speed does it do so? Do different types of intelligence decline at different rates?

In order to delve into these questions, Metafact asked five experts in intelligence, behavior sciences and psychology, 'Does IQ decrease with age?'. Here's what they said…

What is IQ and how is it measured?

"Intelligence is usually measured by a set of tests, for instance, some about language skills, some about non-verbal skills such as solving puzzles, some about how quickly you can complete a task," says Michael Thomas, an expert in psychology and neuroscience from Birkbeck University in England.

"Your intelligence will be the average of your scores across these tasks, compared to how well other people do."

IQ tests assess different abilities such as how well you retain and learn information, your abstract reasoning, and visual-spatial processing.

IQ stands for 'intelligence quotient' and is a score which is standardized relative to other people your age.

If your intelligence is average for your age, your IQ score will be 100. If it is above average it will be above 100, and below average below 100.

Does an individual's IQ change with age?

An individual's IQ does not change with age.

In other words: if you did an IQ test now and then another one in 10 years' time, your IQ score will probably be very similar. This is because IQ is always measured relative to other people your age.

"IQs are always calculated relative to a person's age, whether that age is 10, 15, 25, 50, 72, or 88. So 25-year-olds are compared to other 25-year-olds in terms of the number of items they answer correctly on any given task, just as 50-year-olds are compared to other 50-year-olds," explains Alan Kaufman, an expert in intelligence testing from Yale University in the US.

"For every age group, the average or mean IQ is set at 100. We can't directly compare the mean IQs across the adult age range because – by definition – every group will average 100."

Meiran Nachshon, an expert in psychology from Ben-Gurion University in Israel, agrees, saying:

"IQ indicates the relative positioning of an individual relative to the average. This relative positioning is extremely stable."

To support this, he highlights a publication which found a strong correlation between IQ of people at age ~11 and age ~90.

Does the population's average IQ change with age?

To measure how IQ changes over time, we need to be able to compare the IQ of older people with their younger counterparts.

This is not usually possible because of the reasons described above, instead a different method is needed.

Kaufman explains how this works:

"The first thing we have to do is to find a common 'yardstick' to compare adults. We can compare the performance of 70-year-olds, 60-year-olds, 50-year-olds, 40-year-olds, etc. to the norms (reference group or standards) established for young adults.

"In my research, we define young adults as about age 30 (usually ages 25–34). In that way, young adults will have an average IQ of 100 because that is the way the norms are developed. When we compare adults across the life span to young adults that will tell us how IQ changes as we get older."

Kaufman says that when these tests are done, "[a] clear decline [in IQ] is evident".

Not all types of intelligence decline at the same rate

IQ tests measure many kinds of intelligence and groups them together.

"Global IQ is an amalgam of different kinds of intelligence, the most popularly studied being fluid intelligence and crystallized intelligence which together – along with abilities called working memory and processing speed – are combined to yield global or Full Scale IQ," Kaufman says.

"Fluid intelligence or fluid reasoning reflects the ability to solve novel problems, the kind that aren't taught in school," he explains, "whereas crystallized intelligence or crystallized knowledge measures learning and problem solving that are related to schooling and acculturation."

These different types of intelligence show different patterns as you get older.

Crystallized intelligence "averages 98 at ages 20–24, rises to 101 by ages 35–44, before declining to 100 (ages 45–54), then 98 (55–64), then 96 (65–69), then 93 (70–74), and 88 (75+)," says Kaufman.

Fluid intelligence drops much more quickly. Kaufman reveals that it "peaks at ages 20–24 (100), drops gradually to 99 (25–34) and 96 (35–44) before starting a rollercoaster plunge to 91 (45–54), 86 (55–64), 83 (65–69), 79 (70–74), and 72 (75+)."

Thomas says: "The fastest response times you will ever have are in your mid-twenties, but (so long as you don't develop dementia), your knowledge of vocabulary will increase throughout your life.

"Into your late sixties, most cognitive skills relying on things you have learned (so-called crystalized knowledge) either increase or are pretty resilient. The speed with which you can do things can decline."

The takeaway:

Your individual IQ will not change as you age, but on average our intelligence does decrease with age.

May the facts be with you!

Article based on expert answers to this question: Does IQ decrease with age?

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