Researchers say their analysis of more than a million people challenges previous assumptions that mental speed peaks at age 20.

As humans age, it takes longer to react to changes in the environment, or to stimuli.

This slowing of response time starts from the age of about 20, gradually continuing to slow as people get older.

The new study found that although response times started to slow after 20, this could be due to people being more cautious, and could also be down to slower processes not linked to decision-making, such as time taken to press a key.

Researchers found the mental process of making a decision did not start to slow down until age 60, after which it progressively declined.

The researchers from Heidelberg University in Germany looked at data from more than one million people who took part in an online experiment that measured their reaction times to a mental task.

The participants were shown a selection of words and images on a screen and asked to put them in one of two categories, for example good or bad, by pushing a key.

Despite a widespread belief in age-related decline in mental processing speed, the findings highlight how for much of life this is not likely to be the case, the researchers say.

“Our results indicate that response time slowing begins as early as age 20, but this slowing was attributable to increases in decision caution and to slower non-decisional processes, rather than to differences in mental speed,” the researchers wrote in the journal Nature Human Behaviour.

“Slowing of mental speed was observed only after approximately age 60.

“Our research thus challenges widespread beliefs about the relationship between age and mental speed.”