Child support applies until your child turns 18 years old. However the assessment may be extended in some circumstances.
Your child support assessment may be extended to the end of the school year if your child turns 18 while at secondary school. For that to happen, the other parent must apply to have the assessment extended when the child is 17 and prove that the child will be in full time secondary education on their 18th birthday.
If you have a court order that ends on the child's 18th birthday the other parent will need to apply to the court if they want the order to be extended. Alternatively you may determine a private arrangement with the other parent for the support of the child until such time as the child is able to support themselves.