The education laws of each country establish starting cutoff dates for compulsory education which could, potentially, affect students' academic achievement. Specifically, they could be detrimental for those students whose birthday is just before the school entry cutoff, as they will be the youngest in their classroom, while it could boost the academic performance of those whose birthday is just after (the eldest ones). The present research paper analyses this issue in respect to students' academic achievement using census and longitudinal data for the most populated region of Spain (Andalusia) with a regression discontinuity methodology. Our results show that those students who are the youngest in the classroom due to this school entry cutoff present lower academic achievement compared to the eldest students, an effect which is reduced while students advance in their education. This motivates policy recommendations aimed at checking students' cognitive skills prior to their access to compulsory education and at letting parents choose whether or not to delay their children's access to school in case they are not yet ready.