In terms of arrears, you may still be obligated to pay those until all the arrearages are satisfied, even if the child has turned 18 yrs old.
For "current" child support, it depends in part on what the language of the last relevant court order and/or settlement agreement says re: child support termination. If it says something along the lines of, "Child support shall terminate upon the child turning 18 yrs old," you are likely no longer obligated to pay c/s. In which event, and this is just hypothetical, as I am assuming certain facts, you may want to file a petition to modify with the focus being to terminate the "current" child support payments.
I suggest that you speak with a family law attorney who can better advise you based on your particular facts and whatever relevant court orders or agreements exist.
Your arrears are due until the total arrearage amount is satisfied. Your current child support obligation depends on the language in your child support order regarding the termination of child support. If you are having trouble determining your obligation, you may want to consult with an attorney before your next child support payment date.