A child in UK law is someone who is under 18 years of age. This is reflected in a number of legal provisions, in particular the Convention on the Rights of the Child which defines a child as:
Everyone under the age of 18 unless under the law applicable to the child majority is attained earlier
In UK law, majority is attained (i.e. you become an adult) at the age of 18.
The law relating to child trafficking mirrors this:
Art 4 (d) Council of Europe Convention on Action against Trafficking in Human Beings 2005:
d. Child shall mean any person under eighteen years of age
So when we are talking about child trafficking we are referring to children under the age of 18.
There will be times when a person is only suspected or recognised as being a victim of trafficking once they are over 18 despite the fact that they had been trafficked as a child. Such individuals are still child victims of trafficking.