In this paper, we estimate the effect of pupil's relative age within the first grade of primary school on mathematics and reading test scores at age 15. The main objective is to evaluate the long-term causal effect of relative age in the first grades of primary school on pupil's test in 16 different countries. We use the national rule for admission to primary school to construct the predicted relative age of each pupil. We find that relative age at the start of primary school has a significant positive effect on test scores in about one-half of the considered countries and regions. Moreover, we identify some of the channels through which the effect occurs. (Contains 11 tables, 2 figures and 11 notes.)