Gemara

(Mishnah): A nine-year old (forbids at the beginning, but not at the end).

Contradiction (Rav Zvid bar R. Oshaya's Beraisa): If one gave a Ma'amar to his Yevamah, and later his nine-year old brother had Bi'ah with her, she is forbidden.

Answer: His Bi'ah forbids even at the end. His Ma'amar forbids only at the beginning.

(Beraisa): They made Bi'ah of a nine-year old like Ma'amar of an adult;

96b (Mishnah): If Levi, a nine year old, had Bi'ah with his Yevamah, and later his nine-year old brother Reuven had Bi'ah with her, this forbids her to Levi;

R. Shimon says, it does not forbid her;

If a nine-year old had Bi'ah with his Yevamah Leah, and then with her Tzarah, this forbids Leah to himself;

R. Shimon says, it does not forbid her.

(Gemara - Beraisa - R. Shimon (to Chachamim)): If Levi's Bi'ah is considered Bi'ah (he acquired her), Reuven's Bi'ah has no effect! If Levi's Bi'ah is not Bi'ah, the same applies to Reuven (so his Bi'ah should not forbid her to Levi)!

(Mishnah): A nine-year old had Bi'ah with his Yevamah Sarah, married Leah after he became an adult, and then died. If he did not have Bi'ah with Sarah in adulthood, Sarah does Chalitzah, not Yibum. Leah may do Chalitzah or Yibum;

R. Shimon says, the Yavam may do Yibum or Chalitzah with either wife he wants, and does Chalitzah to the other.

Question: The Bi'ah of a nine-year old should be like a Ma'amar of an adult, and Leah should be forbidden to do Yibum (like the Tzarah of a Yevamah who received a Ma'amar - 31b)!

Answer #1 (Rav): Chachamim did not make Bi'ah of a nine-year old like an adult's Ma'amar.

(Shmuel and R. Yochanan): It is like a Ma'amar!

Question: The above question is difficult for Shmuel and R. Yochanan!

Answer #2: Tana'im argue about whether or not the Tzarah of Eshes Shnei Mesim may do Yibum. The Tana of the Mishnah (31b) decrees lest people do Yibum to both. He taught about an adult Yavam; the same applies to a minor.

Our Tana holds that Bi'ah of a nine-year old is like a Ma'amar, but we do not decree lest people do Yibum also to the Tzarah. He taught about a minor because he was discussing minors, but the same applies to an adult.

Kidushin 19a - Question (Reish Lakish): Can a man be Meya'ed (marry) his Amah (Yisraelis slave) to a son who is a minor?

Answer #1 (R. Zeira - Beraisa): "A man (...who has adultery is killed)", but not a minor. "That will have adultery with a man's wife" excludes a minor's wife.

If Yi'ud to a minor works, a minor can have a wife!

Objection (and Answer #2): If Yi'ud to a minor is invalid, we would not need a verse to exclude a minor's wife! Rather, this shows that Yi'ud to a minor is valid!

Rejection (Rav Ashi): No, the verse excludes a boy above nine who did Yibum;

Since the Yevamah falls to him to Yibum, and his Bi'ah is considered Bi'ah, one might have thought she becomes his full wife, and others are killed for adultery with her. The verse teaches that this is not so.

Nidah 45a (Mishnah): If a Yavam nine years and one day old did Yibum, it takes effect. He may not divorce her until he becomes an adult.

Question: It does not suffice to divorce her when he becomes an adult!

(Beraisa): Yibum of a nine-year old acquires mid'Rabanan like a Ma'amar.

If a Yavam wants to divorce after a Ma'amar, he must give a Get to undo the Ma'amar and do Chalitzah to remove her Zikah. Likewise, if a nine-year old did Yibum, his Get (when he matures) must be accompanied by Chalitzah!

Answer (Rav): It means, if he had relations with her after he matured (which is full Yibum), he can divorce her (without Chalitzah).

Rishonim

Rif and Rosh (Yevamos 31a and 10:7): If one gave a Ma'amar to his Yevamah, and later his nine-year old brother had Bi'ah with her, she is forbidden. Bi'ah of a nine year-old forbids even at the end.

Nimukei Yosef (DH Masnisin Choletzes): Even though Bi'ah of a nine year-old is Bi'ah, he cannot Mekadesh or acquire until he brings two hairs. Regarding a Yevamah, since she is Zekukah to him, Chachamim made his Bi'ah like a Ma'amar.

The Rif and Rosh (ibid.) bring the Gemara in Nidah verbatim.

Rambam (Hilchos Yibum 1:16): We have a tradition that the Bi'ah of a boy above nine years is Bi'ah. Therefore if a Yavam above nine had Bi'ah with his Yevamah he keeps her. He cannot do Chalitzah until he matures and we check that he has two hairs. His Bi'ah does not acquire totally. Therefore, she is not permitted to a stranger until he does Bi'ah or Chalitzah after he matures.

Rambam (5:18): Bi'ah of a Yavam above nine years old is like a Ma'amar of an adult. It does not acquire totally.

Magid Mishneh: The Ramban says that mid'Oraisa he acquires her fully, except that others are not Chayav Misah for Bi'ah with her (Kidushin 19a). Chachamim made his Bi'ah only like a Ma'amar. The Tosefta (11:13) says that when Bi'ah needs Da'as (e.g. Kidushin), Bi'ah of a lunatic or minor is not considered Bi'ah. When Bi'ah does not need Da'as (e.g. Yibum), it is considered Bi'ah. Some disagree.

Minchas Bikurim (Tosefta 11:13): The Seifa teaches that when Bi'ah does not need Da'as, i.e. Yibum, Bi'ah of a Cheresh or lunatic is Bi'ah. It does not discuss a minor, who cannot fully acquire a Yevamah.

Rambam (19): If a nine year-old had Bi'ah with his Yevamah, he disqualified her to the brothers. If an adult Yavam gave a Ma'amar to a Yevamah (Leah) and later his nine-year old brother had Bi'ah with her or a Tzarah, he forbids Leah to his brother, as if an adult gave a Ma'amar after his brother's Ma'amar.

Rambam (20): If Levi, a nine year old, had Bi'ah with his Yevamah Leah, and later his adult brother did Bi'ah or Chalitzah with her or gave to her a Ma'amar or Get, or had Bi'ah with a Tzarah, he forbids Leah to Levi. Likewise, if Levi later had Bi'ah with a Tzarah, or if his nine-year old brother had Bi'ah with Leah or a Tzarah, this forbids Leah to Levi, just like a Ma'amar after a Ma'amar.

Rambam (21): If a Yavam above nine years did Yibum and they did not have Bi'ah after he matured (i.e. full Yibum), she needs a Get and Chalitzah. She needs a Get because his Bi'ah is like a Ma'amar. She needs Chalitzah to permit her to a stranger, since the Bi'ah did not acquire her totally. If they had Bi'ah after he matured, he can divorce her (without Chalitzah).

Rambam (Hilchos Isurei Bi'ah 3:1): If a man had Bi'ah with a minor's wife, even if she is a Yavam who did Yibum with a minor above nine, he is exempt. If he was Mezid, he is lashed mid'Rabanan.

Noda bi'Yhudah (EH 54 DH uv'Guf): The Rambam holds like Rashi (Kidushin 19a DH demid'Oraisa), that mid'Oraisa he acquires her. If he acquires only mid'Rabanan, she is still a Yevamah l'Shuk, and there are Makos mid'Oraisa for her! It is improper to say that the Rambam says that one, i.e. another brother is lashed for her mid'Rabanan, or that a stranger is lashed mid'Rabanan if an adult brother later did Chalitzah.

Rebuttal (Or Some'ach Yibum 1:17 DH v'Zeh): The Rambam (Yibum 2:!8) holds that one is lashed mid'Oraisa for Yevamah l'Shuk (like other Chayavei Lavin) only amidst Kidushin. The Rambam cannot hold that the Yibum acquires mid'Oraisa. If it did, the Drashah would exempt from Misah, but there would be a Chiyuv Kares and Korban, like the Rambam says about Chamoso after the wife died. Since the Rambam does not obligate Kares and Korban, this shows that he holds that the Yibum is only mid'Rabanan.

Note: The Rambam does not discuss punishment for Eshes Katan through Yi'ud. In Hilchos Avadim (4:7) he says that Yi'ud can be to an adult son of the master. The Stam Gemara (Kidushin 19a) expounds "Eshes Ish" to exempt for a minor's wife resulting from Yi'ud. Rav Ashi says that it refers to one who did Yibum with a nine year-old. The Rambam rules unlike the Stam Gemara, so he must expound like Rav Ashi. If the Yibum was not mid'Oraisa, no verse would be necessary! The Or Some'ach must say that the verse is a mere Asmachta.

Tosfos (Kidushin 19a DH umid'Oraisa): Yibum of a minor only forbids to the brothers. Mid'Oraisa it has no effect.

Tosfos Rid (Kidushin 19a DH Hacha): If the nine year-old did not have Bi'ah after maturity he cannot divorce her without Chalitzah. This shows that he did not acquire her totally. Chachamim made his Bi'ah like a Ma'amar only to disqualify her to the brothers.

Ramban (Yevamos 61b DH v'Ha): The Ra'avad holds that Bi'ah of a nine year-old does not acquire fully so that his wife would become forbidden to him through Zenus. This is wrong. Rav says that Bi'ah of a Cheresh totally acquires, even though he lacks Da'as. The same applies to a nine-year old.

Note: Perhaps the Ra'avad learns from the fact that the Gemara never discusses warning a minor's wife about seclusion. We expound (Sotah 27a) "Ish Ish" to teach that Beis Din warns the wife of a Cheresh, lunatic, bewildered man, or a man who went overseas or is imprisoned. Everywhere else we include a minor with a Cheresh and lunatic!

Poskim

Shulchan Aruch (EH 167:1): Bi'ah of a Yavam above nine years old is like a Ma'amar of an adult.

Shulchan Aruch (2): If a Yavam above nine did Yibum he cannot exempt her with a Get alone unless he has relations with her after he matured. Otherwise, she needs also Chalitzah.

Shulchan Aruch (EH 170:16): Bi'ah of a Yavam above nine years old is like a Ma'amar of an adult. If a nine year-old had Bi'ah with his Yevamah, he disqualified her to the brothers. If Levi, a nine year old, had Bi'ah with his Yevamah Leah, and later his adult brother did Bi'ah or Chalitzah with her or gave to her a Ma'amar or Get, or had Bi'ah with a Tzarah, he forbids Leah to Levi. Likewise, if Levi later had Bi'ah with a Tzarah, or if his nine-year old brother had Bi'ah with Leah or a Tzarah, this forbids Leah to Levi, just like a Ma'amar after a Ma'amar. If an adult Yavam gave a Ma'amar to a Yevamah (Leah) and later his nine-year old brother had Bi'ah with her or a Tzarah, he forbids Leah to his brother, as if an adult gave a Ma'amar after his brother's Ma'amar.

Beis Shmuel (12): According to Rashi, the Yavam acquires her mid'Oraisa, so he inherits her. Tosfos holds that he does not acquire mid'Oraisa, so he does not inherit her.