It has been hypothesized that individual differences in cognitive ageing might in part be based on the relative preservation of speed of information processing. However, the biological foundations of processing speed are not understood. Here we compared two groups of non-demented older people who had relatively similar IQs at age 11 but differed markedly in non-verbal reasoning ability at age 70: ‘cognitive sustainers’ (n= 25), and ‘cognitive decliners’ (n= 15). Using an event-related fMRI design, we studied the BOLD response while they performed an inspection time task. Inspection time is a two-alternative forced choice, backward masking test of the speed of the early stages of visual information processing. Inspection time has a well-established, significant association with higher cognitive abilities. The group of cognitive sustainers showed a pattern of BOLD activation–deactivation in response to inspection time stimulus duration differences that was similar to a healthy young sample [Deary, I.J., Simonotto, E., Meyer, M., Marshall, A., Marshall, I., Goddard, N., Watdlaw, J.M., 2004a. The functional anatomy of inspection time: an event-related fMRI study. NeuroImage 22, 1466–1479]. The group of cognitive decliners lacked these clear neural networks. The relative preservation of complex reasoning skills in old age may be associated with the preservation of the neural networks that underpin fundamental information processing in youth.