Most mental health conditions start by adolescence, with half established by age 14 years. In 2022, 18% of children aged 7 to 16 years and 22% of young people aged 17 to 24 years had a probable mental health condition. Rates of diagnosis have increased. Diagnosed anxiety, depression, attention, and eating disorders doubled between 2003 and 2018. COVID-19 restrictions likely had an additional impact. In addition to the impact on children and their families, mental health conditions have a large economic cost and impact on public services. National lifetime loss of earnings for children in England who experienced anxiety problems age 14 and also failed to achieve at least five GCSEs amounts to more than £850 million. Prevention of mental health conditions in children and young people will have life-long benefits to individuals and society, including economic benefits. Mental health is affected by social and economic factors. However, intervention research has typically focused on psychological therapies and interventions targeting individual behaviours, rather than the social determinants of mental health conditions.