Because of the regulations concerning school entry in most school systems, a child's date of birth may significantly affect his or her age at entry into school, and thus their first experience of schooling. Using data from the Programme for International Student Assessment (PISA), this paper provides a comparative analysis of the impact of a student's month of birth on cognitive and non-cognitive outcomes. It describes school regulations regarding school entry in over 45 countries and economies, and discusses the reasons why a student's date of birth may have consequences on his or her performance in school. The results show that a student's month of birth has consequences on performance in the three main domains assessed by PISA, and also on the student's progress through education, as those children who were the youngest in their grade cohort at entry into school were more likely to have repeated a grade in primary school. This paper also shows that, in several school systems, being the youngest in the school-entry cohort has an impact on self-confidence, notably on self-perceived competence and self-efficacy, and also on future education outcomes. These results call for raising awareness amongst educators and parents of the initial disadvantage experienced by the youngest children in their first years of school. The paper concludes with a review of existing recommendations to reduce age-related effects on education outcomes.