In recent years, almost all children below school age in Western industrialized countries have some experience of attending day care institutions. However, the age at which children enter day care and therefore the overall time spent in day care varies substantially. We investigate the potential impact of later day care entry on the social and emotional behaviour of children, one important aspect of non-cognitive skills. Based on the English sample of the Millennium Cohort Study, we analyse the effects on children's development at the age of five and seven, using propensity score techniques. We find clear evidence of effects on children's development at the age of seven: Later day care entry increases children's peer-problems and reduces prosocial behaviour. We find that boys with low educated mothers and from families with a household income below the poverty line are most strongly affected. (author abstract)