18 if they were in full time education, but there have been cases where the child has gone to the family court for maintenance for when they are in university. The child is the one who has to sue for maintenance though, the mother is out of the picture as they are an adult. A high earning wealthy father is the only one likely to have to pay for his adult child in this way. See a family solicitor.
It doesn't have to be the child Jolly - the resident parent can make a court application for maintenance to continue beyond eighteen as described below, but the application must be made before the child turns eighteen. www.fishermeredith.co.uk/legal-advice/3674
Jolly is right. It's a weird quirk of Englihs law. If parents stay together and choose not to pay a penny towards university they are allowed to. If the parents are apart the child can apply! I have never understood that.
Scottish law is different again as I think parents have a duty to support children to age 25 or something.
Our divorce court order says I pay all university costs whoever the children live with. Lucky me.
Daughter's went to '18 or the end of full time education' in the agreement because we expected her to need support through university. From 18 it was paid to her, and that was fine, as her birthday is in June so she was through with school and getting ready to start uni at that time. Might have been a bit uncomfortable if she'd had a September birthday, as I used maintenance to pay school fees and not having it would have left me in difficulty.