Previous studies has found that the youngest students perform more poorly, on average, than older peers in school. This phenomenon is known as the relative age effect (RAE). In Norway, the age difference within a grade year can be up to 12 months. All Norwegian students participates annually in national reading, numeracy and English tests in 5th, 8th and 9th grade, which tests students’ basic curriculum skills. We apply population data on national tests to study RAE. Specifically, we aim to investigate: The linear effect of RAE on national tests, the ratio of RAE to the grade effect in grade 8/9 in numeracy and reading and lastly how RAE changes from grade 5 to 9 in numeracy. This study applies ordinary least square regression to estimate the linear effect of RAE, sharp regression discontinuity design to estimate RAE and grade effect on grade 8/9 in reading and numeracy. We incorporate vertical linking of the numeracy tests to investigate how RAE changes over grade years. In accordance with previous findings, we confirm that RAE is strongest in grade 5 and declines over grade years. We found that RAE has a larger impact than the grade effect on national reading and numeracy tests in grade 8/9. These results suggests that RAE has a stronger impact on national test proficiency than the amount of years spent in school at this point of the educational track.