This article draws on the Millennium Cohort Study (MCS) to examine parent ratings of social, emotional and behavioural difficulties and prosocial behaviour in pre‐ and mid‐adolescents. A series of mixed‐design ANOVAs yielded interesting results. Parent ratings of emotional difficulties in girls increased as they moved from pre‐ to mid‐adolescence whereas for boys the reverse was found. Peer problems were found to be on the rise, whereas prosocial skills decreased for 14‐year‐olds. Most importantly, significant associations were found between socio‐economic measures (that is, family income and parent education) and ratings across the domains of the Strengths and Difficulties Questionnaire, highlighting the socio‐economic specificity of behaviour and well‐being in adolescents. These findings have significant implications for understanding trends in young people’s social behaviour and emotional well‐being from pre‐ to mid‐adolescence within their socio‐economic context.