This article examines the impact of within-class age differences on educational outcomes, using students' birth months in Madrid's primary schools as a natural experiment. Employing a regression discontinuity design, we analyze third-grade students to investigate these age-related effects. Additionally, we explore whether early childhood education attendance works as a mitigating factor. Results indicate that relatively older students achieve higher scores in both Language and Mathematics and have lower grade retention rates. However, this gap is attenuated among students who attended childhood education for two years or more. These novel results highlight the importance of early childhood education in reducing natural inequalities that may persist over time.