Ordinarily, child support is ONLY modified if one party files a motion. And there is only a presumption of changing the child support if the new financial circumstances would cause a change of 15% increase or decrease in the amount paid. However, a recent Court of Appeals case, Seay v. Seay, 2012-CA-001786 (July 5th 2013) highlights one even that mandates a review of the child support regardless. The change can only date back to the date the motion was filed.
When a child is emancipated, then the child support must be reviewed as of the date of emancipation. A child is emancipated by operation of law once they turn 18 years of age OR at the completion of the school year if still enrolled in the year that they turn 19 years of age. This emancipation date CAN be changed by an agreement in the underlying divorce or custody case.