As human age increases so does the incidence of a variety of changes in their brains and central nervous systems (CNS) which, increasingly, affect their mental efficiency. Models for "cognitive ageing" have distinguished between two possible aetiologies for these CNS changes: "normal" or "usual" changes that, eventually, occur in all humans, at rates determined by both genetic and environmental factors and the accumulation, over a lifetime, of pathologies and biologically significant accidents coupled with an accelerating increase in the numbers and severity of pathologies in old age. This paper compares the relative proportions of variance in cognitive performance within an elderly population that are associated with their current ages from birth, their distances from death, the specific causes of their deaths and their self-reports of their health status, of the number of different clinical conditions from which they suffer and their recent and long-term use of medical care and of their difficulties with activities of everyday life. Age at the time of assessment accounts for up to 20 % of variations in cognitive performance between individuals. All other factors are significant, but surprisingly modest predictors of current cognitive ability. These results allow re-assessment of distinctions between models for cognitive ageing.