In order to take FMLA leave to care for a son or daughter who is 18 years of age or older, the adult child must have a disability and be incapable of self-care due to that disability.The son or daughter must also have a serious health condition for which he or she needs care.n eligible employee is entitled to take FMLA leave to care for a son or daughter with a serious health condition who is 18 years of age or older and incapable of self-care because of a disability regardless of when the disability commenced. 3.