This cohort study investigated the effect of early life nutrition on later performance in developmental milestones and cognitive tests. Structural equation modelling was carried out on follow-up data from a national birth cohort of 5362 children born in 1 week of 1946 in Britain. Children who were breastfed longer showed earlier developmental milestones. After adjusting for sex, social group and milestones, those children scored higher in verbal tests. Path analysis showed that breastfeeding may have an effect even into adult life. The chain of path coefficients from breastfeeding to developmental milestones at early ages, to cognitive scores at ages 8–15 years, to reading tests at age 26 years and to memory and visual tests at age 43 years were significant in females but not in males.