Recent studies demonstrated that children who are relatively young within a school year show more behaviour problems, are at greater risk of being diagnosed with attention-deficit/hyperactivity disorder (ADHD) and treated with stimulants compared to their older classmates. In this paper we examine the association of age at school entry and behavioural problems for a German sample of elementary school children, who were enrolled in 2009 (N = 928). We used teacher ratings on the Strengths and Difficulties Questionnaire (SDQ) as well as the prevalence of ADHD diagnosis and medication. Our analyses showed that those children with a relatively younger age at school entry were judged more hyperactive than their older peers when sex was controlled for. After stratifying for sex, the effect of age at school entry on hyperactivity was now only significant for boys. No association with age at school entry could be found for the other SDQ-subscales as well as for ADHD diagnosis and medication. Although we could only partially demonstrate a significant relationship between age at school entry and behavioral problems in German elementary school-children, understanding these associations may help to consider children’s maturity differences when evaluating the child’s behavior.