Recent social and educational policy debate in the UK has been strongly influenced by studies that have found children’s cognitive developmental trajectories to be significantly affected by the socio-economic status of the households into which they were born. Most notably, using data from the 1970 British cohort study, Feinstein (2003) concluded that children from less-advantaged backgrounds who scored high on cognitive tests at 22 months had been overtaken by age seven by children from more advantaged origins, who had scored lower on the baseline test. However, questions have been raised about the methodological robustness of these studies, particularly the possibility that their key findings are, at least in part, an artefact of regression to the mean. In this paper, we assess growth mixture models as an alternative approach for identifying and explaining cognitive developmental trajectories in children which is robust to regression to the mean. We apply this approach to children’s cognitive test score data from the Millennium Cohort Study. Our findings provide no support for the contention that more initially able children from disadvantaged backgrounds are ‘overtaken’ in cognitive development by less initially able children from more affluent backgrounds. We do, however, find that cognitive developmental trajectories are related to socio-economic status, such that initial class-based inequalities increase over time.