You must have taken steps in the past to legitimate the child OR must have established a bona fide parent-child relationship and be able to prove one or the other. IF the requiremenst were met in the past, then the adult son or daughter would qualify now. for I-130 purposes.
INA 101(b) [This is the definition of child for immigrant visa purposes.]
(b) As used in titles I and II-
(1) The term "child" means an unmarried person under twenty-one years of age who is-
(A) a child born in wedlock;
(B) a stepchild, whether or not born out of wedlock, provided the child had not reached the age of eighteen years at the time the marriage creating the status of stepchild occurred;
(C) a child legitimated under the law of the child's residence or domicile, or under the law of the father's residence or domicile, whether in or outside the United States, if such legitimation takes place before the child reaches the age of eighteen years and the child is in the legal custody of the legitimating parent or parents at the time of such legitimation;
(D) a child born out of wedlock, by, through whom, or on whose behalf a status, privilege, or benefit is sought by virtue of the relationship of the child to its natural mother or to its natural father if the father has or had a bona fide parent-child relationship with the person;
additional sections relate to adoptions...