If the child support order mandates he pay her until she is 21, then the issue becomes whether the limitation also mandates that the daughter be living with the custodial parent at the time. Child support is for the benefit of the child and is to be paid to the custodial parent to defray costs. The issue may become moot if she is able to support herself.  Most child support after 18 is only allowed for educational purposes; if she is still going to school, she should still be eligible.