In the United States, the length of time someone must pay child support is governed by the state, not the federal government. The length of child support obligations is dependent on the relevant laws of their particular state.
States generally have similar laws regarding child support. In most cases, parents do not have to pay child support once the child becomes a legal adult. If the child has special needs, however, that may require a parent to continue payments beyond this point. State laws also stop generally stop requiring support payments if someone else adopts the child in question or if a court grants the child emancipation from his parents.