Short answer: Under the ACA, plans that cover dependent children must allow them to stay on a parent’s plan until they turn 26, regardless of whether the child is married, in school, living at home, or financially independent.
Federal law lets adult children remain on a parent’s health plan until age 26, with no other conditions: the child can be married, not a student, not living with the parent, and financially independent, and still qualify (though the child’s own spouse and children generally aren’t covered). Coverage usually ends at the end of the month the child turns 26, which is itself a qualifying event that opens a special enrollment period for the child to get their own coverage. Some states extend dependent coverage past 26 under specific conditions, so check your state’s rules.