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).1
The Youth Justice Legal Centre describes any person aged under 18 as a child.
In the criminal justice system in England and Wales, a child can sometimes mean someone aged 10-132 and a young person can sometimes mean someone aged between 14-17.3