101 Facts About Ageing #31: Reaction Speed Slows With Age

As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.

Simple reaction time (the time it takes to take a single action in response to a single stimulus, like pressing a button in response to an image appearing on a screen) increases by about 0.5-1 millisecond per year in most studies, with the average reaction time of a 20 year-old being measured at around the 200 millisecond mark. However, many real-life scenarios require an individual to recognise one of multiple stimuli and make an appropriate decision. For example, a driver might have to decide whether to break or swerve in order to avoid an accident. Measurements of this choice reaction time have found it to be much more heavily influenced by age, declining at a rate of around 2-3 milliseconds per year. The average reaction time itself at a given age is very variable and depends in part on the type and number of stimuli.

As can be seen from the figure above, while there is a tendency for reaction time to increase with age, it is very variable between individuals, with many 60 year-olds having equivalent or even better reaction times than some 20 year-olds. It isn’t clear what causes this variation, though higher levels of education correlate with better reaction times.

Reaction time incorporates many separate processes that are affected by ageing: alertness and attention, transmission time between sensory and motor cortex, response generation in the motor cortex, and conduction velocity of motor neurons. In the case of choice reaction time, the speed of discrimination between multiple stimuli is also an important factor. The myelin sheath, which wraps around some nerve fibres and increases the speed of electrical signals, suffers some level of deterioration during ageing, as does the motor cortex itself. However, it isn’t clear to what extent these changes affect reaction times.