The question should be whether your child is emancipated, as upon emancipation your child support obligation would terminate. If your child is over the age of 18 and no longer a full time student (without disability or other legitimate reason why he/she cannot be self-sufficient) there is a presumption of emancipation.
If the child's primary custodian will voluntarily agree that the child is emancipated and your child support obligation is terminated the two of you can execute a Consent Order confirming those facts and file the same with the court. If the other party will not cooperate, you have to file a Motion with the court seeking (1) to emancipate the child and (2) to terminate your child support obligation.
Kenneth A. White, Esq.
The Answer provided was based on the limited information provided, and represents information based on the law in general, not a legal opinion that can be relied upon. Before a formal legal opinion can be offered I would need an opportunity to review all possible relevant facts and circumstances. You cannot rely on the advice of an attorney given over the internet. The exact facts of your sitaution, including facts which you have not mentioned in your question, may completely change the opinion that is being offered. Please be aware that the above comments are neither protected by attorney-client privilege, nor may the same be the basis for a malpractice lawsuit.