What are the Customs for adopting a 21 yr old boy?

Without going through the case meniton by you.

Only real blood relation can give any person in adoption.

Step mother can only become guardian.

About custom,

Customs are the local law of the land and prevail over any other law.

Plz read the books and ask ur professors, We can help u to solve any case but not the basic law else we will become tutor only.
Regards and Good luck
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Section 10 of HMA is repeated for your information :-

10      Persons who may be adopted

No person shall be capable of being taken in adoption unless the following conditions are fulfilled, namely:-

            (i)        he or she is Hindu;

            (ii)        he or she has not already been adopted;

(iii)       he or she has not been married, unless there is a custom or usage applicable to the parties which permits persons who are married  being taken in adoption;

(iv)      he or she has not completed the age of fifteen years, unless there is a custom or usage applicable to the parties which permits persons who have completed the age of fifteen years being taken in adoption.