Mass. Gen. Laws ch. 120 § 16
Every person committed to the department until the age of 18 as a delinquent child, if not already discharged, shall be discharged upon reaching the person's eighteenth birthday. Every person committed to the department until the age of 19 as a delinquent child, if not already discharged, shall be discharged upon reaching the person's nineteenth birthday. Every person committed to the department until the age of 20 as a delinquent child, if not already discharged, shall be discharged upon reaching the person's twentieth birthday. Every person committed to the department until the age of 21 as a youthful offender, if not already discharged, shall be discharged upon reaching the person's twenty-first birthday. The department may continue to provide, for any person covered in this chapter under 22 years of age, specific educational, rehabilitative or transitional services and supports, under conditions agreed upon by both the department and such persons and terminable by either. Such services must be offered prior to the person's discharge date as provided herein; however, a person may request and the department shall consider any such request for the services described after the person's effective date of discharge, even where the person has previously declined services or services were terminated for non-compliance, and may reach an agreement with the person, terminable by either, to provide the services described herein until such person attains the age of 22.
Mass. Gen. Laws ch. 120, § 120:16