An adoption of a child may be decreed when written consents to adoption have been executed by:
(1) the birth mother;
(2) the husband of the birth mother if the husband is the presumed father of the child under 40-6-105;
(3) any other person whose parental rights have been established by a court;
(4) the department or an agency that has custody of the child and the authority to place the child for adoption;
(5) the legal guardian of the child if both parents are dead or their rights have been judicially terminated and the guardian has authority by order of the court appointing the guardian to consent to the adoption;
(6) the child, either in writing or in court, if the child is 12 years of age or older unless the child does not have the mental capacity to consent.