Background: Younger children in a school class have higher rates of mental health problems if admission to primary school occurs once a year. This study examines whether this relative age effect also occurs if children are admitted to school continuously throughout the year.Methods: We assessed mental health problems based on parent-reports (using the Child Behavior Checklist, CBCL) and on professional assessments, among two Dutch national samples of in total 12,221 children aged 5-15 years (response rate: 86.9%).Results: At ages 5-6, we found a higher occurrence of mental health problems in relatively young children, both for mean CBCL scores (p = 0.017) and for problems assessed by child health professionals (p <0.0001). At ages 7-15, ...