It has been put forward by many empirical studies that the month of a student's birth is crucial in relation to their school achievement. Within this context, this study aimed to investigate whether there was a relative age effect on the mathematics and science achievement of fourthgrade Turkish students and how the relative age effect changed after the implementation of the 4+4+4 system. In addition, the impact of the interaction of the students' ages with other factors of gender, home resources, and the number of books owned on student achievement was analyzed. The participants of the study comprised 10297 students, who were in the age range determined within the context of some criterion among Turkish students who participated in Trends in International Mathematics and Science Study (TIMSS) 2011 and TIMSS 2015. Multivariate multilevel modeling was utilized in the data analysis. At the student level, the interaction of the age variable with gender, the home resources, and the number of books owned was analyzed. The region variable was included in the model at the school level. In this context, it is determined that the relative age effect was observed in student achievement before and after the 4+4+4, but this effect was found to be higher in 2015. In other words, it can be expressed that the students' ages placed them at a disadvantage in 2015. Moreover, the interaction between age and home resources was determined to have a negative effect while the interaction between age and number of books owned had a positive effect on student achievement.