Objective To estimate the impact on early development of prematurity and summer birth and the potential ‘double disadvantage’ created by starting school a year earlier than anticipated during pregnancy, due to being born preterm. Design, setting and patients We investigated the impact of gestational and school-entry age on the likelihood of failing to achieve a ‘Good Level of Development’ (GLD) on the Early Years Foundation Stage Profile in 5-year-old children born moderate-to-late preterm using data from the Born in Bradford longitudinal birth cohort. We used hierarchical logistic regression to control for chronological maturity, and perinatal and socioeconomic factors. Results Gestational age and school-entry age were significant predictors of attaining a GLD in the 10 337 children who entered school in the correct academic year given their estimated date of delivery. The odds of not attaining a GLD increased by 1.09 (95% CI 1.06 to 1.11) for each successive week born early and by 1.17 for each month younger within the year group (95% CI 1.16 to 1.18). There was no interaction between these two effects. Children starting school a year earlier than anticipated during pregnancy were less likely to achieve a GLD compared with (1) other children born preterm (fully adjusted OR 5.51 (2.85–14.25)); (2) term summer births (3.02 (1.49–6.79)); and (3) preterm summer births who remained within their anticipated school-entry year (3.64 (1.27–11.48)). Conclusions These results confirm the developmental risks faced by children born moderate-to-late preterm, and—for the first time—illustrate the increased risk associated with ‘double disadvantage’.