The Children Act 1989 and the Children Act 2004 define a child as being a person up to the age of 18 years. However, the Children Act 2004 states that in certain circumstances safeguarding, protection and cooperation between services may be continued through to a young person's 19th birthday or beyond.

In this guideline, 'children' are aged 12 years or younger.

(A similar definition is given in guideline NG53 (Mental health problems in people with learning disabilities: prevention, assessment and management) and NG11 (Challenging behaviour and learning disabilities: prevention and interventions for people with learning disabilities whose behaviour challenges).