IF YOU ARE over 20, look away now. Your cognitive performance is probably already on the wane. The speed with which people can process information declines at a steady rate from as early as their 20s.
A common test of processing speed is the “digit symbol substitution test”, in which a range of symbols are paired with a set of numbers in a code. Participants are shown the code, given a row of symbols and then asked to write down the corresponding number in the box below within a set period. There is nothing cognitively challenging about the task; levels of education make no difference to performance. But age does. Speed consistently declines as people get older.
Why this should be is still a matter of hypothesis, but a range of tentative explanations has been put forward. One points the finger at myelin, a white, fatty substance that coats axons, the tendrils that carry signals from one neuron to another. Steady reductions in myelin as people age may be slowing down these connections. Another possibility, says Timothy Salthouse, director of the Cognitive Ageing Laboratory at the University of Virginia, is depletion of a chemical called dopamine, receptor sites for which decline in number with advancing age.
Fortunately, there is some good news to go with the bad. Psychologists distinguish between “fluid intelligence”, which is the ability to solve new problems, and “crystallised intelligence”, which roughly equates to an individual’s stock of accumulated knowledge. These reserves of knowledge continue to increase with age: people’s performance on vocabulary and general-knowledge tests keeps improving into their 70s. And experience can often compensate for cognitive decline. In an old but instructive study of typists ranging in age from 19 to 72, older workers typed just as fast as younger ones, even though their tapping speed was slower. They achieved this by looking further ahead in the text, which allowed them to keep going more smoothly.
What does all this mean for a lifetime of continuous learning? It is encouraging so long as people are learning new tricks in familiar fields. “If learning can be assimilated into an existing knowledge base, advantage tilts to the old,” says Mr Salthouse. But moving older workers into an entirely new area of knowledge is less likely to go well.This article appeared in the Special report section of the print edition under the headline “Old dogs, new tricks”