Gemara

(Mishnah - R. Gamliel): If she does Mi'un (if not, she waits until she grows up, and then her older sister is exempt).

Question (R. Elazar): What is R. Gamliel's reason?

Does he hold that Kidushin of a minor is Teluyah (tentative), and when she matures, her Kidushin matures (becomes mid'Oraisa) with her, even without Bi'ah?

Or, does he hold that Kidushin with his Yevamah's sister his Yevamah from Yibum and Chalitzah? If so, this is only through Bi'ah with his wife when she is an adult.

Answer (Rav): He holds that Kidushin with Achos Yevimto exempts his Yevamah. This is only through Bi'ah after she matures.

Question (Rav Sheshes - Beraisa): If one is Mekadesh a minor, the Kidushin is Teluyah.

Suggestion: This means that when she grows up, it becomes mid'Oraisa even without Bi'ah.

Answer (Ravin brei d'Rav Nachman): No, the Kidushin is Teluyah: it becomes mid'Oraisa only if they have Bi'ah after she matures;

She says, he has power over me, and I have power over him.

Question: Rav holds that it becomes mid'Oraisa even without Bi'ah!

(Rav): If a minor did not do Mi'un, and she married David (another man) after she matured, she does not need a Get from David;

(Shmuel): She needs a Get from David.

Suggestion: They argue about when she did not have Bi'ah with her first husband (Reuven) after maturing.

Answer: No, the case is, they had Bi'ah.

Question: If so, why doesn't Shmuel require a Get?

Answer: He holds that when Reuven has Bi'ah, he relies on the initial Kidushin (and does not intend to Mekadesh through Bi'ah).

Objection: Rav and Shmuel already argued about this!

(Rav): If a man was Mekadesh a woman on condition, and did Nisu'in without mentioning the condition (and the condition was not fulfilled), she needs a Get.

(Shmuel): She does not need a Get.

Rav requires a Get. Since he did Nisu'in, he pardoned his condition;

Shmuel does not require a Get. When he has Bi'ah, he relies on the initial Kidushin.

Answer: It was necessary for them to argue in both cases.

Had they argued only about the condition, one might have thought that only there Rav holds that she is married, since he pardoned the condition, but regarding a minor who matured, he would agree with Shmuel;

Had they argued only about a minor who matured, one might have thought that only there Shmuel holds that she is not married to the first husband, but regarding a condition, he would agree with Rav!

Nidah 46a (Rav Dimi of Neharda'a): The Halachah is, (a girl above 12 may not do Mi'un even if we see that she does not have hairs now;) we are concerned lest she brought hairs after 12 years, and they fell out.

This is only if she had relations with her husband after 12, for then it is a Safek mid'Oraisa. If not, we are not concerned.

Rishonim

Rif and Rosh (13:7): The Gemara did not say what is the Halachah (if Kidushin matures automatically with her, or only through Bi'ah after maturity). Some say that it is a Safek, and we are stringent. Even though we answered the question from the Beraisa (which says that the Kidushin is Teluyah), we do not rely on the answer. If she matured and became Mekudeshes to another man before having Bi'ah with her husband, she needs a Get from both. This is good reasoning. The Gemara (Nidah 46a) supports it. Rav Dimi rules that if a girl is above 12 and we see that she does not have hairs now, we are concerned lest she brought hairs after 12 years and they fell out (so she may not do Mi'un). This is only if she had relations with her husband after 12 years, for then it is a Safek mid'Oraisa. If not, she may do Mi'un, for it is only a Safek mid'Rabanan, so we are lenient. However, in any case we check her, and if she brought hairs she may not do Mi'un, even if she did not have Bi'ah after 12. Mid'Rabanan, she needs a Get; if another man was Mekadesh her, it takes effect. However, if she had had Bi'ah with her husband after she matured, his Kidushin became mid'Oraisa, and she does not need a Get from the Mekadesh, like Rav taught. The Beraisa that says that Kidushin are Teluyim refutes Shmuel.

Question (Ra'avad (brought in Sefer ha'Zechus and Rosh): What is the Rif's doubt? Ravin gave a proper answer (the Kidushin becomes mid'Oraisa only if they have Bi'ah after she matures)! Rav Sheshes accepted it, and said 'I used to think that I am better (wiser) than he (Ravin), but I see that he is (able to explain the Beraisa according to Rav) better than I'.) Surely Rav agrees that she needs a Get and may not do Mi'un! If she did not do Bi'ah and accepted Kidushin from another, surely Rav requires a Get from the Rishon mid'Rabanan and from the Sheni mid'Oraisa!

Question (Ba'al ha'Ma'or): What is the Rif's doubt? Rav and Shmuel argue only about when there was Bi'ah. The Rif himself says that the Beraisa refutes Shmuel when there was Bi'ah! If there was not Bi'ah, all agree that she needs a Get from both! Surely we rely on Ravin's answer, for it is a stringency (if she did not have Bi'ah and accepted Kidushin, we require a Get).

Nimukei Yosef (DH Gemara): The Gemara suggested that the Kidushin is Teluyah, i.e. it becomes mid'Oraisa without Bi'ah. This is like one who is Mekadesh to take effect in 30 days if she will not retract within this time. Even if the money was consumed before 30 days it takes effect, because she is obligated to return the money (if she retracts). The same applies here (even though she was a minor when she took the money), because Chachamim instituted marriage mid'Rabanan. Or, perhaps it becomes mid'Oraisa only through Bi'ah. The Halachah follows Rav against Shmuel. She needs a Get from the Sheni mi'Safek only if there was not Bi'ah.

Rambam (Hilchos Gerushin 11:4): If a girl married mid'Rabanan had Bi'ah after 12 years, since Bi'ah acquires mid'Oraisa (if she brought hairs), she may not do Mi'un.

Rambam (6): If a girl matured and did not do Mi'un, even if she did not have Bi'ah with her husband after 12 years she may not do Mi'un, for she is an adult. She needs a Get mid'Rabanan; the marriage cannot be mid'Oraisa. Therefore, if another man was Mekadesh her, it takes effect.

Poskim

Shulchan Aruch (155:20): If a girl (married mid'Rabanan) had Bi'ah with her husband after 12 years, this is a Safek mid'Oraisa. If we would know that she brought hairs, surely he had Bi'ah intending for Kidushin, and if another man was Mekadesh her she would not need a Get from him.

Shulchan Aruch (21): If she brought two hairs after 12 years and did not have Bi'ah after this, this is Safek Kidushin. If he wants to send her away she needs a Get mid'Rabanan. If he did not divorce her and she became Mekudeshes to another man, she needs a Get from both.

Taz (11): The Safek is whether or not her Kidushin matures with her.

Gra (71): The Rif and Tur say that it is a Safek. Other Poskim say that there is no Safek. Rav agrees that she can do Mi'un after bringing hairs as long as there was no Bi'ah. The Gemara in Nidah (52a) supports them (it says that the Halachah follows R. Yehudah, who permits Mi'un until there is much hair, but not if she had Bi'ah after maturity). In Chulin 26b, Rav himself says that a Stam Mishnah is an individual's opinion, but Chachamim (R. Yehudah) say that there is a girl old enough to do Chalitzah who can do Mi'un.

Bach (DH Hevi'ah): Kidushei Ketanah is void. The Safek of the Tur (and Shulchan Aruch) is whether or not it becomes Kidushin mid'Rabanan when she matures. This is unlike the Nimukei Yosef.

Rebuttal (Beis Shmuel 32): Surely the Safek is whether or not it is Kidushin mid'Oraisa! R. Gamliel says that the Kidushin exempts her sister from Yibum or Chalitzah! However, even though the Safek was mid'Oraisa, we are lenient not to be concerned lest she brought hairs and they fell (Nidah 46a). This is not because it is a Sefek-Sefeka (perhaps she did not bring and perhaps the Kidushin does not mature), for here one of the Sefekos is a Safek in Halachah. Rather, when we know that there was no Bi'ah the Safek is only mid'Rabanan. Normally, if they are living together we assume that there was Bi'ah.