A third of those over 80 years of age are likely to have dementia, the lack of a cure requires efforts directed at prevention and delaying the age of onset. We argue here for the importance of understanding the cognitive ageing process, seen as the decline in various cognitive functions from adulthood to old age. The impact of age on cognitive function is heterogeneous and the identification of risk factors associated with adverse cognitive ageing profiles would allow well-targeted interventions, behavioural or pharmacological, to delay and reduce the population burden of dementia. A shift away from binary outcomes such as dementia assessed at one point in time in elderly populations to research on cognitive ageing using repeated measures of cognitive function and starting earlier in the life course would allow the sources of variability in ageing to be better understood.