Mental health includes our emotional, psychological, and social well-being.  It is about feeling good about life and this affects how we think, feel, and act. It also helps determine how we handle stress, relate to others, and make choices.  Mental health is important at every stage of life, from childhood and adolescence through adulthood.
See Child and Adolescent Mental Health Services (CAMHS) and CAMHS Intellectual Disability Service (CAMHS ID).