A child is a person aged under 18 unless under the law applicable to the child, majority is attained earlier, according to the United Nations Convention on the Rights of the Child (UNCRC).
It is the settled position of the law in the UK that anyone under 18 is a child, and should be referred to as such. This aligns with the definition used in the Children Act 1989,
The Youth Justice Legal Centre therefore describes any person aged under 18 as a child.