Journal article: This paper performs a precise quantification of date of birth effects throughout individuals’ schooling and working life using high-quality French data. 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 cohort and explains why date of birth effects persist into adulthood.