Professor Arthur Shimamura. of the University of California at Berkeley, says there are three main ways in which mental function changes. The first is mental speed, for example how quickly you can react to fast-moving incidents on the road. Drivers in their late teens react quickly but tend to drive too fast, while the over sixties are more cautious but react more slowly. The near-inevitable slowing with age also partly explains why soccer players are seen as old in their thirties, while gulf professionals are still in their prime at that age. This type of mental slowing results from a reduction in the efficiency with which the brain’s neurons work.