According to some opinions, an adopted convert can go by ben-his adopted father's name (in terms of calling to the Torah for instance), would this apply only to people adopted under a certain age?
If you had an adult convert (let's say 20) that was "adopted" so to speak by a Jewish family, including being treated like a son and looked after as you might look after your adult child, could the convert take the name of the father? ie Reuven ben Ploni
There are opinions that permit this and they don't differentiate by age.
R Doniel Neustadt writes (here) that
The poskim disagree as to whether an adopted child should be called to the Torah as the son of the adoptive father. Harav S.Z. Auerbach rules that if the biological father’s name is known, then the child should be called to the Torah by that name. If the biological father’s name is not known, then he may be called to the Torah as the son of the adoptive father.
din.org.il writes
Concerning an adopted child who was converted, Rav Moshe Feinstein (Shut Iggros Moshe, Yoreh De’ah 1:161; see also Even HaEzer 1:99) ruled that the child can be called up as the son of his adoptive father. [...] For calling up to the Torah, and where an element of shame for the adopted child is involved, it seems appropriate to use the adoptive father’s name (but see Shut Minchas Yitzchak 4:49; 5:46; 6:151, who prohibits this practice).
See the relevant footnotes in the original for other opinions as well.
My impression is that a family that decides to look after a 20-year-old (without developmental challenges) is a good friend and important influence, not an "adoptive parent."
Adoptive parents (or biological parents of converts) are referenced in a ketubah as "hamegadlo" -- the one who raised him (or her). I just don't see how coming into someone's life at age 20 can be considered "raising them."
Would the cutoff then be halachic majority, i.e. bar/bat mitzvah? Or the age at which someone could theoretically strike it off on their own, let's call it 18 in America today? (The latter is what 21st Century American rabbinic courts will use for child support.) I don't know. But 20 -- minimum age for the censuses in the Bible for warfighters -- sounds wrong.
