Although students generally start their elementary school education at a given legal age, there are considerable age differences between classmates. Within a class of timely enrolled children, these age gaps can be up to eleven months - a significant amount of time especially for the youngest cohorts. Our study contributes to the debate in educational economics and related social science fields to what extent these relative age gaps influence academic outcomes. We examine the existence, magnitude, and duration of relative age effects in South Korea for various school subjects. Our results show that relative age effects exist among Korean elementary school students and that these effects are more pronounced in science-related subjects than in language subjects. We also find that female students tend to be less affected by relative age effects than their male peers. Importantly, however, we show that relative age effects generally decline after fourth grade and disappear completely by the start of secondary education.