Recent epidemiological evidence suggests that individuals who have higher levels of mental ability in youth experience a slower cognitive decline as they grow old. In a sample of 3,263 Newcastle residents, average scores on a vocabulary test (Raven's 1965 ‘Mill Hill A') did not vary, while average scores on a test of fluid mental ability (the Heim, 1970, AH 4 (1) group intelligence test) sharply declined with age from 49 to 92 years. In young adults, Mill Hill A scores are good proxies for AH 4 (1) scores. This relationship allowed individuals' youthful AH 4 (1) test scores to be estimated from their current, unchanged, Mill Hill A scores so that age-related changes in AH 4 test scores over the adult life-span could be estimated and compared between high and low ability groups, men and women, and individuals of different levels of socio-economic advantage. The cross-sectional estimated rate of age-related decline in general mental ability was found to be the same for people of all levels of ability and socio-economic advantage, and not to differ between men and women.