Adult adoptees are those who are adopted over the age of 18. Obviously this kind of adoption works very differently from that of infant or child adoption and every state has different laws governing the legality of adult adoption. For an adoption to take place, the adult adoptee will need to give his or her consent. This consent means the adopter will have legal parental responsibility over the adult and both parties agree to be a family, legally.
Outside of the symbolic gesture that an adult adoption can be, it's also a practical option. Adults who were unable to be adopted but became legal adults while in the foster system can opt to have their foster parents adopt them. Other adult adoptees may want to be adopted by their birth parents after conducting a successful search and reunion. Sometimes adult adoptees choose to be adopted as a way to legally bind themselves to an inheritance.
Nebraska and Michigan are the only two states in the United States that do not allow anyone over 18 to be adopted. Other states, like Arizona, have an upper limit of 21 years old. Other states, like Ohio, only permit adult adoption to adults and caregivers if there is an established foster or stepparent relationship between the adoptee and adopters. States will also usually make exceptions for adults with severe mental and physical disabilities.