The relationship between pupil performance and date of birth is a well documented empirical fact. Yet relatively little attention has been given to the institutional features of educational systems that explain why early relative maturity differences between individuals born at different times of the year can have lasting effects on their educational and labour market outcomes. This paper aims at contributing to fill this gap by taking advantage of high-quality French data that allows a precise quantification of date of birth effects throughout individuals’ schooling and working life. With respect to previous studies, our paper innovates by controlling explicitly for the confounding effects that may arise from the correlation between birth seasonality and socio-economic background. Our results indicate that although the test score gap between December and January-born pupils tends to narrow as they grow older, it remains significant until the end of secondary education. In the French context, we show that the combination of extensive use of grade retention and early school tracking is particularly harmful for the youngest pupils in their academic cohort. Being born in December rather than in January doubles the probability of being held back in school and the negative signal associated with grade retention has a strong influence on the type of senior secondary school that students attend after completing junior high school. We argue that both these institutional features explain why date of birth effects persist in adulthood. Our estimates reveal that women and men born in December have a higher probability of holding a vocational qualification and a lower probability of holding an academic qualification than those born in January. Furthermore, men born at the end of the year incur a small but significant penalty in terms of labour market outcomes, in the form of lower wages and higher unemployment rates.