With the Law No. 6287, known as the 4+4+4 regulation in education in public, the age of starting primary school was reduced to 60 months, and this regulation was first implemented in the 2012-2013 school year. This research aims to analyze the effect of the age difference among students who started school in the 2012-2013 school year and whose ages ranged from 60 months to 84 months, based on the 2020 High School Entrance System (LGS) Central Examination data, which they participated in the eighth grade. The research was designed as correlational and causal-comparative research. The students who participated in the 2020 LGS Central Examination from middle schools in Ankara constituted the population of this research, and 9582 students selected from these schools constituted the sample of the research. The school starting ages of these students were grouped, and comparisons were conducted between the LGS Central Exam scores of the groups and the net numbers of correct answers in the subtests of this exam. In this exam, it has been determined that the academic performance of the students in the older age group is higher than that in the younger age groups. The reasons for the difference between the academic performance of the students are listed as the insufficient readiness level of the students in the younger age group when they start primary school, the adaptation problems of the students to the primary school, the reduction of the primary school period from five years to four years, the inability to differentiate and individualize the instruction by focusing on the whole group instruction. According to the results of this research, it is necessary to develop education policies on a scientific basis, age of schooling should be 72 months, primary school education should be five years, and development experts should evaluate the readiness levels of all children at the stage of starting primary school.