Until when does the effect of age on academic achievement persist? Evidence from Korean data

According to an analysis of Korean student panel survey data, monthly differences in age had a significant influence on academic achievement until middle school (lower secondary education). However, this age effect did not persist when students graduated from high school (upper secondary education). Furthermore, some evidence is found that younger students, upon entering high school, were more likely to concentrate on academic studies, and less likely to experience minor distractions, thereby compensating for their poor academic achievement in middle school.