A growing number of studies suggest that relatively young behavior of pupils gives them a much greater likelihood of being diagnosed with a disorder such as ADHD. This 'relative age effect' has also been demonstrated for special educational needs, learning difficulties, being bullied, and so on. The current study investigated the relationship between relative age of pupils in primary education and teachers' perception of their behavior. The study sample included 1973 pupils, aged between 6 and 12. Six linear mixed models were carried out with birth day in a year as predictor variable and 'total problem score', 'problems with hyperactivity', 'behavioral problems', 'emotional problems', 'problems with peers' and 'pro-social behavior' as dependent variables. Random intercepts were added for school and teacher level. Cluster-mean centering disaggregated between-school effects and within-school effects. We found no associations between relative age of pupils and teacher perceptions of their behavior. Several explanations are postulated to account for these findings which contradict prior studies on relative age effects.