(a) A child committed to the department for guardianship, after termination of parental rights, shall remain in the care of the department until he or she attains the age of eighteen years, or is married, or is adopted, or guardianship is relinquished through the court.
(b) A child committed to the department for custody shall remain in the care of the department until he or she attains the age of eighteen years, or until he or she is discharged because he or she is no longer in need of care.