Grade retention has been the focus of the education debate in Spain for decades. On average, more than 30% of students have repeated at least one grade before they finish (or dropout from) their compulsory studies. The present research provides new evidence on this issue by investigating the influence of Spain's school entry age upon students' grade retention. Using data from 15-year-old students who participated in the PISA 2006, 2009, 2012 and 2015 assessments, we implement a regression discontinuity analysis. Our key finding is that students who were born late in the year (younger students) are more likely to repeat a grade. Yet, once they reach secondary education, the disadvantage they suffer due to their younger school starting age seems to disappear. Hence, the key reason why younger students have lower PISA scores than older students in Spain is due to their increased likelihood of repeating a grade, rather than being due to their relative age per se. To avoid these artificial disadvantages of younger students and unfair retention, we suggest that policymakers inform families about this school entry issue and also make the school entry law more flexible. This would facilitate parents of younger children to choose whether to delay their children's school enrolment or not.