Section 19. When an order of the department is confirmed as provided in section eighteen, the control of the department over the person shall continue, subject to the provisions of this chapter; provided, however, unless the person is previously discharged in accordance with section six (e), the department shall discharge the person upon his twenty-first birthday. The department shall have the power, in order to protect children and adolescents, to transfer the custody of any person eighteen years of age or older to the department of correction for placement in an appropriate institution.