["Developmental Science, Volume 29, Issue 5, September 2026. ", "\nABSTRACT\nWhether an optimal school‐entry age exists is widely debated. A sequential cohort longitudinal design was used to investigate how school‐entry deferral affected executive function development, and parent‐reported socio‐emotional and self‐regulatory outcomes in 110 children (majority White) over one academic year. Groups comprised: 35 Deferred School Entrants (Mage = 66 months, 17 females), 31 Non‐deferred School Entrants (Mage = 57 months, 13 females), and 44 Deferred Pre‐schoolers (Mage = 55 months, 22 females). Deferral was associated with improved parent‐reported socio‐emotional and self‐regulatory outcomes, especially in children with initially lower scores, while schooling benefited working memory progress. Further, older children started school with better working memory and inhibitory control relative to younger school entrants, although baseline differences had partly waned over the school year. These results highlight the value of accounting for regulatory skills underlying both cognitive and socio‐emotional outcomes within school entrance age debates. Further research is required to determine if, and how, this might relate to academic outcomes.\n"]