The aim of this study was to examine the contribution of Age of Onset Criterion (AOC) to the diagnosis of Attention Deficit Hyperactivity Disorder (ADHD) and disruptive behavior disorder. For this purpose, a 10-item Likert-type Parent Assessment of Pre-school Behavior Scale (PARPS), developed by the experimenters, was used to examine the presence of ADHD related pre-school behaviors in a sample of 246 children. Factor analysis and correlational analysis imply the continuity of ADHD and disruptive behavior disorder from pre-school to the elementary school years. Discriminant analysis was used to examine the diagnostic capabilities of PARPS, both by itself, and in conjunction with a battery of widely used diagnostic and clinical ADHD and disruptive behavior scales. The results of this study suggest that PARPS is a short and user-friendly scale that can contribute to the examination of the presence of AOC in ADHD and disruptive behavior disorder.