In this article, we exploit the 1947 change to the minimum school-leaving age in England from 14 to 15, to evaluate the causal effect of a year of education on cognitive abilities at older ages. We use a regression discontinuity design analysis and find a large and significant effect of the reform on males’ memory and executive functioning at older ages, using simple cognitive tests from the English Longitudinal Survey on Ageing as our outcome measures. This result is particularly remarkable as the reform had a powerful and immediate effect on about half the population of 14 years olds. We investigate and discuss the potential channels by which this reform may have had its effects, as well as carrying out a full set of sensitivity analyses and robustness checks.