An adult may be adopted, with consent. Most Relevant Massachusetts Statutes:
Title III, Chapter 210
§ 3 § 1. Situations Where Adoption Authorized.
A person of full age may petition the probate court in the county where he resides for leave to adopt as his child another person younger than himself, unless such other person is his or her wife or husband, or brother, sister, uncle or aunt, of the whole or half blood.§ 2. Written consent of certain persons; form of consent; identification of father
A decree of adoption shall not be made, except as provided in this chapter, without the written consent of the child to be adopted, if above the age of twelve; of the child's spouse, if any; § 3. Dispensing with required consent in certain cases
(a) Whenever a petition for adoption is filed by a person having the care or custody of a child, the consent of the persons named in section 2, other than that of the child, shall not be required if:
(i) the person to be adopted is 18 years of age or older;