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.