yes it is possible to adopt a 21 year old, but can you please tell why you would want to adopt a 21 yr old? no need to secure parents' consent, he is already of majority age, it would depend on your reason why you want to adopt.  adoption law was passed to protect minors first and foremost, the welfare of the person to be adopted is the primary consideration

Section 5. Who may be adopted. � The following may be adopted:

(1) Any person below eighteen (18) years of age who has been voluntarily committed to the Department under Articles 154, 155 and 156 of P.D. No. 603 or judicially declared available for adoption;

(2) The legitimate child of one spouse, by the other spouse;

(3) An illegitimate child, by a qualified adopter to raise the status of the former to that of legitimacy;

(4) A person of legal age regardless of civil status, if, prior to the adoption, said person has been consistently considered and treated by the adopters as their own child since minority;

(5) A child whose adoption has been previously rescinded; or

(6) A child whose biological or adoptive parents have died: Provided, That no proceedings shall be initiated within six (6) months from the time of death of said parents.

(7) A child not otherwise disqualified by law or these rules.