California Code, Family Code - FAM § 9320

(a) An adult may adopt another adult who is younger, except the spouse of the prospective adoptive parent, by an adoption agreement approved by the court, as provided in this chapter.

(b) The adoption agreement shall be in writing, executed by the prospective adoptive parent and the proposed adoptee, and shall state that the parties agree to assume toward each other the legal relationship of parent and child and to have all of the rights and be subject to all of the duties and responsibilities of that relationship.