The school entry age was changed from 72 to 66 months in 2012 with a new education system adjustment in Turkey. The aim of the study was to investigate the effects of lowering school entry age on Attention Deficit Hyperactivity Disorder (ADHD) diagnosis and symptom severity. The records of children at first and second grade diagnosed with ADHD according to the DSM-IV diagnostic criteria in the Child Psychiatry outpatient clinic between January and July 2010 (when the old system was in use) and between January-July 2013 (when the new system was in use) were retrospectively screened to create the old-system and new system groups. Among the two groups, T-DSM-IV-S fulfilled by parents and teachers were used to assess symptom severity. The frequency of ADHD and ADHD predominantly inattentive subtype diagnosis we found to be significantly higher among the girls in the new system compared to old system (25.8%, 8.9%, p=0.027 - 56.3%, 0%, p=0.012). Additionally, mother’s subscale scores of T-DSM-IV-S were lower among the children in the new system compared to the ones in the old system. By lowering school entry age due to the new education system, frequency of ADHD diagnosis increased while symptom severity rates decreased among the first-grade girls. Thus, it may be suggested that despite decreased symptom severity, the girls who started school with the new system were diagnosed with ADHD more frequently due to a marked disruption in academic, social, and behavioral functionality associated with insufficient neurodevelopmental maturity.