In Scotland a child is defined in family law as a person under the age of 18 years or over that age and under the age of 25 years when undergoing eduction or training. Unfortunately in E&W there is no such definition and if child maintenance is arranged through a Court order (either by consent or imposed) you will need to check the terms of the order. A Court may extend the term of the order for over 18s in education or training.
For CSA purposes a child is under 16 or between 16 and 19 and in full time, non-advanced education ie not higher than A level standard. However, for student loan purposes a dependent student is defined as under 25 and having not lived independently for at least 3 years.
Under the Children Act 1989 over 18s may apply in their own right for maintenance from parents although this route isn't open if there was a court order in respect of them before their 16th birthday.
Confused?
