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The Mishnah discusses a case in which a nine-year-old boy lives with his Yevamah, and then his nine year-old brother also lives with her. The Tana Kama rules that the woman becomes prohibited to both brothers. Rebbi Shimon rules that she remains permitted to the first Katan.
RASHI asks, why does she remain permitted to the first Katan? When the second Katan lives with her she becomes a Sotah, for she committed adultery with him! Rashi adds that although the adulterer was a Katan, nevertheless his Bi'ah is considered a valid act of Bi’ah, and her act is punishable with Misah. Rashi answers that when she lives with the second Katan, she does so b'Shogeg (inadvertently), unaware that the boy is not her husband, or unaware that the act is prohibited. Therefore, she does not become a Sotah, and she remains permitted to her husband.
Rashi apparently follows his own opinion as expressed in Kidushin (19a, DH d'mid'Oraisa;) that an act of Yibum performed by a nine-year-old is considered a valid Yibum mid'Oraisa. Consequently, his act of Yibum creates a bond of Kidushin with the woman, who will become a Sotah if she lives with another man after she does Yibum with the nine-year-old.
We examine the sexuality of a nine year old and the implication of his marrying and yibum status.