One of the important differences between educational systems from different countries is the age at which students are placed into separate tracks. We examined the effects of the age at which tracking occurred on student achievement in a comparative perspective, making use of recent waves of three internationally standardized student assessments (PISA, TIMSS, and PIRLS). In order to control for unobserved national heterogeneity, we adopted a differences-in-differences approach, in which we controlled secondary school results for differences already present in primary school (i.e. before the introduction of tracking). The results indicate that early tracking has a negative effect on mean performance of students, particularly in the domain of literacy. Moreover, by separating out groups with different abilities, it is shown that early tracking has a very strong negative effect on low achieving students, suggesting that disadvantageous peer- and environmental effects in the lower tracks may have detrimental consequences on students' academic achievements. By contrast, a null effect on the group of top achieving students was found, suggesting that comprehensive systems can equally challenge high performers to learn at a high pace.