Within the Mental Capacity Act's code of practice, 'children' refers to people aged below 16.  'Young people' refers to people aged 16-17.  This differs from the Children Act 1989 and the law more generally, where the term 'child' is used to refer to people aged under 18.

The summary linked from this page looks at the few parts of the Act that may affect children under 16 years of age and explains the position of young people aged 16 and 17 years and the overlapping laws that affect them.

Mental Capacity Act - How it applies to children and young people - a Summary