Setting the age for starting elementary students at 60 months old has been a hotly debated topic in Turkey since the Grand National Assembly passed an education reform bill as Law No. 6287, known to the public as the "4+4+4 regulation." At the outset of its implementation, students who started first grade in school year 2012-2013 had an age distribution in the range of 60 to 84 months. The present research aims to determine the effect of this age difference among students in the same grade level on their subsequent development. Thus, the study is a longitudinal and causal comparative research since comparison of the two age groups on both readiness for the school and the academic development throughout the four assessments within six months intervals was investigated. The results indicate that students younger than 69 months are disadvantaged against their older peers in all the specified fields and subfields during all the periods studied.