Background: Compulsory school entrance age and admission policies differ across countries and educational systems, and there is a continuing debate on the question whether and how the age at school entry affects various student outcomes. Aims: This study explored the relation between school entrance age and school achievement among students attending fourth (ages 10-11) and eighth (ages 14-15) grade of Croatian primary schools. Sample: Participants were 44,479 fourth- and 43,338 eighth-grade students from all Croatian primary schools. Students were divided into groups of younger and older school entrants based on the difference between the year of birth and the year of school entry. Methods: Differences in school achievement between students who were younger and those who were older when they entered school in subjects covered by the curriculum for respective grades were examined among fourth- and eighth-grade students. Two achievement measures were used for each subject-objective test results obtained during a national examination and final school marks appointed by teachers. Results: Among fourth-grade students, older school entrants on average performed slightly better than the younger ones in all the subjects, irrespective of the achievement measure used. However, these differences in achievement are very small, and the effect of school entrance age on achievement is very weak. In the eighth-grade sample, younger and older school entrants did not differ in their achievement in the majority of subjects. Conclusions: Students who are older when they enter school perform slightly better than their older classmates in the lower grades of primary school, but these differences in achievement are very small and are probably no longer present by the end of primary schooling.