305 ILCS 5/2-11: "Family Unit": Husband and wife and a child or children under age 21.

Spouses and parents of children under age 21 constitute "legally responsible relatives" or "responsible relatives" wherever those terms may be used in this Code in respect to their support obligation enforceable by court action or an administrative order of the Illinois Department, as provided in Article X, or enforceable through other State or Federal laws.

The support obligation of other persons defined as "legally responsible relatives" in this Section prior to October 6, 1969 for aid extended to their dependents prior to that date shall remain unimpaired.