I actually think they just need to reach the age of 18 (not over). Here is how it was stated in my settlement.:
Duration of Payments: The Husband's obligations under this paragraph shall commence upon the effective date of this Agreement and shall continue until the emancipation, as defined hereinafter, of the children in question: 1. The child attaining the age of eighteen (18) years or completion of a high school education, whichever later occurs; 2. The child maintaining a full time residence outside of the home of the Custodial Parent, exclusive of the child's residence at a secondary boarding school, camp, or similar facility; 3. The child obtaining full time employment, exclusive of the child's employment during school vacation periods; 4. The child marrying; 5. The child entering the Armed Forces; or 6. The death of the child.