The aim of this study was to investigate secular trends regarding proportions of individuals showing cognitive decline, stability, or gain between ages 70 and 79 years. For this purpose we analyzed data from three Swedish birth cohorts, born 1901-02, 1906-07, and 1930, and four cognitive measures (i.e. verbal meaning, figure identification, logical reasoning, and spatial ability). Using the standard error of measurement (SE_m) at baseline (i.e. 70 years of age), participants were categorized as showing cognitive decline (if decreasing by more the 1 SE_m between ages 70 and 79), gain (if increasing by more than 1 SE_m) or otherwise evincing stability. χ²-tests indicated statistically significant differences between the cohorts for all four cognitive measures. For all cognitive measures earlier born cohorts consisted of a smaller proportion of individuals showing cognitive decline, and a larger proportion showing cognitive gain compared to later born cohorts. Possible explanations for these results are discussed in terms of cohort differences in selective survival and/or cognitive reserve.