As humans grow older, some important cognitive functions deteriorate, even in the absence of dementia [6]. Abilities such as memory, reasoning, and spatial ability all decline, on average, with age [7]. Not everyone declines equally: some people show marked cognitive declines as they grow older, whereas others maintain cognitive skills relatively well in old age [7]. Normal cognitive ageing is a continuous trait. It is rare to be able to study this phenomenon, because there are few samples that have had cognitive testing performed more than once and with a sufficient amount of time between the measurements. We previously described the distribution of normal cognitive change across almost 70 years, from age 11 to age 79 [8]. Before interventions can be found to enhance the lives of older people, the determinants of individual differences in general and specific aspects of cognitive ageing must be discovered.
