Depends on the state. see link below
As a GENERAL statement - Most support orders cease upon the child's graduation from high school or when they legally become an adult. Some settlements will call for support up through and including...
This depends on factors such as the language of the court order and the laws of the State having jurisdiction over the matter.
age 18 see link below
Florida removed the law establishing an age when child support ends in October of 2010. See related links below.