IF YOU ARE over 20, look away now. Your cognitive performanceis 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.
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