US civil and family laws allow you to establish legal parent-child relationship. US immigration law also will admit this relationship, but you won't get
any immigration benefit including GC through this relationship because it was happened after you have turned 18 (The law is not even using 21 here, but 18).
any immigration benefit including GC through this relationship because it was happened after you have turned 18 (The law is not even using 21 here, but 18).