Age at entry to motherhood is increasingly socially polarised in the UK. Early childbearing typically occurs among women from disadvantaged backgrounds relative to women with later first births. The Millennium Cohort finds differentials in their children's development, cognitive and behavioural, at age 5, by mother's age. These could be due to difficulties facing immature mothers, but much of it is attributable to young mothers’ social origins, or inequalities apparent at the age 0 survey, which may also have had earlier origins. The developmental penalty left to be attributed to the mother's age per se is, at most, modest.