The aim of this study is to determine school readiness of primary school students in mixed-age classrooms. The sample of this study, using embedded mixed method design, consists of 909 first grade students in primary school and 30 primary school teachers determined by stratified purposeful sampling method. In this study, "Primary School Readiness Scale" and "semi-structured interview form" were used for data collection. Descriptive and differential statistics were used for quantitative data analysis, and descriptive analysis technique was used for qualitative data analysis. As a result of the study, first grade students of 60-65 months old demonstrated a medium level of school readiness; while the students of 66-71 and 72-84 month old demonstrated a high level of school readiness. In interviews, teachers stated that students' school readiness increased with age. Considering the individual differences of the students, the appropriate age for students to start school should be determined with school readiness test.