From: Rabbi Moshe Sofer
To: Rabbi Yosef Yoel Deutsch
The Chasam Sofer rules that the sale of a cow to a nine-year-old gentile boy is void if the boy has not reached intellectual maturity, since for Bnei Noach majority is determined by comprehension rather than age, and the mechanism of da'as acheres makneh does not apply to non-Jews. Even if the boy were mature, the validity of situmta for transactions with minors who are not typical merchant parties is doubtful. He recommends having a non-Jew blemish the calf so it may be slaughtered, and rejects any suggestion that bechor in our times is only rabbinic.
Source: Shu"t Chasam Sofer YD §317