In Arizona, the obligation to pay child support continues until the child is 18 or graduates high school (whichever comes later), but not past age 19 (unless the child has special needs that will prevent him/her from becoming independent). You didn't indicate whether your child was currently attending (or enrolled in) high school. If so, then the child support would continue; however, if the child is 18 and has graduated, then it is time to stop the withholding from your paychecks. This can be done by having both parents sign a Stipulation to Terminate Order of Assignment, or if the other party won't sign, you can file a Request to Terminate Order of Assignment, and a hearing might be necessary (e.g. if you owe arrears, the court might continue to have your paychecks garnished until the arrears are paid in full).
I'd recommend speaking with an attorney in the county where the order was issued to discuss your options.
** This response is provided as general information only and does not constitute legal advice or otherwise create an attorney client relationship. **