Gemara

34b - Version #1 (Shmuel): Any woman who had Bi'ah must wait three months before marrying someone else (to avoid doubts about who the father is if she will gave birth nine months later), except for a minor who converted or was freed from slavery. A minor Yisraelis must wait.

Shmuel already taught that a minor need not wait after doing Mi'un, but she must wait if she received a Get. Here he discusses when she was Mezanah.

Chachamim decreed that a minor must wait (after Zenus, even though she cannot be pregnant) lest an adult not wait.

Question: We do not make such decrees!

(Mishnah): If they are minors, they return to their husbands immediately.

Answer (Rav Gidal): That was like a Hora'as Sha'ah (a special ruling for an urgent need at the time). It is not common that wives get mixed up (therefore, we did not decree).

Version #2 (Shmuel): A minor Yisraelis need not wait.

He teaches about a minor who had Zenus. This is uncommon, so we do not decree due to an adult.

41a (Mishnah): No woman may get engaged or married for three months, whether or not she is a virgin, whether she was divorced or widowed, from Nisu'in or Kidushin;

R. Yehudah says, if she was only Mekudeshes she may get married. If she was married, she may become Mekudeshes. This does not apply to Arusos in Yehudah, since they are intimate with their husbands;

R. Yosi says, all women may get engaged, but a widow must wait until the Aveilus (mourning) over her husband ends.

42b (Rav Asi): R. Yochanan ruled like R. Yosi because an individual disagrees with him:

(Beraisa - R. Meir): A woman must wait three months before remarrying, (even) in the following cases: she was constantly going to her father's home, or was quarreling with her husband; her husband was imprisoned, or too old to have children, or sick; she was sick, or miscarried after her husband died, or her womb was uprooted; she was too old to have children, or a minor, or an Ailonis, or sterile.

R. Yehudah (some texts - R. Yosi) permits her to marry immediately.

(R. Chiya bar Avin): R. Yochanan retracted.

(Rav Yosef): If he retracted, it was due to the following Beraisa:

(Beraisa - R. Yishmael, son of R. Yochanan ben Brokah): Chachamim in Kerem b'Yavneh require all to wait three months.

Question (R. Yirmeyah): How could R. Yochanan say that the Halachah follows R. Yosi? R. Yochanan said that the Halachah follows a Stam Mishnah!

(Mishnah): Any woman may not get married or engaged for three months, whether or not she is a virgin.

Answer (R. Avahu): This Stam Mishnah is followed by a Machlokes. In such a case, the Halachah is not like the Stam.

Kesuvos 60b (Rav Nachman citing Shmuel): The Halachah follows decrees of R. Meir.

Rishonim

Rif (10a) and Rosh (3:10): Even though a Beraisa requires minors to wait, that is a decree. It is not common that wives get mixed up. Therefore, in our Mishnah Chachamim did not decree. The Halachah follows R. Yosi against R. Yehudah, so one who was seduced or raped need not wait. Some say that the Halachah follows R. Yehudah. I do not know their source. Our Stam Mishnah is even like R. Yosi. R. Yosi permits only one who was Mezanah knowingly, for she turns over to avoid pregnancy. A Mishnah (41a) forbids any woman to get engaged or married within three months, even a virgin from Kidushin. Also this is even like R. Yosi, for it does not discuss Zenus.

Nimukei Yosef (Sof DH Tanya): Zenus, rape and enticement are not common (therefore, we do not decree about them).

Rosh (ibid.) and Ba'al ha'Ma'or: R. Meir is very stringent. He requires Havchanah even if she cannot give birth. The Halachah follows him against R. Yehudah, who is lenient in such cases. All the more so the Halachah does not follow R. Yosi, who is even more lenient! We follow the second version of Shmuel, which exempts from Havchanah only a minor who had Zenus, but not an adult. Shmuel holds like R. Yehudah, but not for his reason. Rather, Shmuel decrees Havchanah after Zenus lest one not wait after Nisu'in. Shmuel does not decree about Zenus of minors, for this is uncommon.

Question (Tif'eres Shmuel 1): The Gemara said so before we knew that Shmuel holds like R. Yosi, that women turn over after Zenus. In conclusion, we decree to require Havchanah after Zenus due to Nisu'in. We do not make this decree after Havchanah after Zenus of a minor (even Nisu'in of a minor is only a decree due to adults, since the minor cannot be pregnant)!

Answer (Korban Nesan'el 400): Perhaps the Rosh did not say so because we find that Chachamim made Gezeirah li'Gzeirah in Havchanah, e.g. after Zenus of a sterile woman.

Rif (14a): 'Too old' refers to a woman who married (for the first time) after 40, or waited 10 years after being widowed or divorced before deciding to remarry.

Rashi (Kesuvos 60b DH Harei): It is uncommon for a woman to get pregnant if she constantly goes to her father's home, or quarrels with her husband...

Nimukei Yosef (DH Tanu): The Beraisa discusses a woman who cannot be pregnant because she was in her father's house or did not have relations due to quarrels (for the last three months).

Rif and Rosh (4:26): The Halachah follows R. Meir, because R. Yochanan retracted. Also, the Halachah follows decrees of R. Meir.

Nimukei Yosef (ibid.): When a Stam Mishnah is followed by a Machlokes, the Halachah is not necessarily like the Stam, but it can be.

Rambam (Hilchos Gerushin 11:20): Chachamim decreed that even a woman who cannot give birth must wait 90 days before remarrying, even if she is a minor or very old or sterile or an Ailonis, even if her husband was overseas or in prison, and even if she is a virgin widowed or divorced from Kidushin.

Poskim

Shulchan Aruch (EH 13:1): Chachamim decreed that a woman must wait 90 days before remarrying even if she cannot give birth, even if she was widowed or divorced from Kidushin, even if she is a minor or very old or sterile or an Ailonis, even if her husband was overseas or in prison, and even if she is a virgin widowed or divorced from Kidushin.

Beis Yosef (DH Kosav ha'Rivash): The Rivash (202) requires waiting even if her previous husband was castrated.

Teshuvas Radvaz (1:264): Tosfos says that Havchanah is only mid'Rabanan. The Rambam says that 'Chachamim decreed... even if she cannot give birth' connotes that when she can give birth it is mid'Oraisa.

Pischei Teshuvah (1): The Mishneh l'Melech (Melachim 8:6) and others hold that the Rambam agrees that it is always mid'Rabanan.