DO ISURIM COME IMMEDIATELY UPON REACHING THE AGE OF ADULTHOOD? [maturity:years]
Gemara
33b - Mishnah: If two brothers were Mekadesh two sisters, and at the time of Chupah they switched wives, they are liable for Eshes Ish, Eshes Ach, Achos Ishto, and if they were Nidos, for Nidah.
34a - Question: Who is our Tana, who obligates for Isur Kolel, Isur Mosif and Isur Bas Achas?
Answer #1 (Rav Yehudah): It is R. Meir, who is Mechayev five Chata'os and one Asham for a Tamei who ate Chelev that was Nosar from a Korban in Reshus ha'Rabim (he took it from Reshus ha'Yachid) on Shabbos that was Yom Kipur.
Answer #2: We need to say only that our Tana obligates for Isur Bas Achas, like R. Shimon.
Question: Granted, the Isurim of Eshes Ish, Eshes Ach and Achos Ishto can come simultaneously if one Shali'ach for both brothers gave Kidushin money for both, at once, to the girls' Shali'ach. But how can the Isurim of Nidah come b'Vas Achas with all of these?
Answer (Rav Amram): If the girls were spurting Dam Nidah when the boys finished 13 years (Bar Mitzvah), all the boys' Isurim come together. If the girls finished 12 years when they were spurting, all their Isurim come together.
Nidah 48b (Beraisa - R. Yehudah): Women do not check a girl (for hairs) during the Perek.
He holds that during the Perek is like after the Perek, i.e. hairs brought then are Simanim;
(Beraisa - R. Shimon): Women check her during the Perek.
He holds that during the Perek is like before the Perek, i.e. hairs are not Simanim. We check in order that if we find the same hairs after the Perek we will know that they are not Simanim.
Rishonim
Rif (Yevamos 39b): All agree that if a boy brought two hairs and he is 13 years and a day, he is an adult. This is a Siman.
Rambam (Hilchos Ishus 2:1): A girl is a minor for 12 full years even if she brought many hairs. If she brought two hairs below and she is 12 years and a day, she is a Na'arah.
Rambam (10): A boy is a minor for 13 full years, even if she brought many hairs. If he brought two hairs below and he is 13 years and a day, he is an adult.
Rosh (Nidah 6:3): R. Tam explains that R. Yehudah and R. Shimon argue about the last day of the 12 years [and a day]. The Halachah follows R. Yehudah (hairs then are Simanim).
Question #1 (Tosfos Yevamos 34a DH mi'Toch): How can all the Isurim come together when they become adults? The boys cannot be Mekadesh until after they are Bar Mitzvah!
Answer #1 (and Question #2 - Rashba 34a DH Hachi): We must say that the Kidushin money was given beforehand. However, when a Nochri is Mekadesh to take effect after conversion, only R. Meir says that it works. We established our Mishnah to be like R. Shimon!
Answer #1 (to Question #2 - Mishneh l'Melech Ishus 4:7 DH Katan): Here, even Chachamim agree that the Kidushin takes effect, because no action needs to be done. It takes effect automatically after time.
Rebuttal (Keren Orah Yevamos 34a DH Gemara): Even R. Meir says that Kidushin works only when the Mekadesh was a Bar Da'as at the time. Here, all agree that it does not work!
Kovetz Ha'aros (29): This is why Tosfos did not say that the problem is Davar she'Lo Ba l'Olam. However, the Rashba and other Rishonim hold that a minor's act can take effect if an adult's Ko'ach is involved, e.g. if it will be Chal when he is an adult.
Question #3: Why are the Isurim Chal immediately when the day begins? They are not adults until they bring hairs after 13. Hairs from before do not help!
Answer #1 (Rashba 34a DH Hachi): We hold like Rav, who says that if a girl brings Simanei Bagrus on the day of Bagrus, we say that she brought them at the beginning of the day (Kidushin 79a). The same applies here.
Answer #2 (to all questions - Tosfos): Some texts say that the boys' Isurim come at once if the girls were spurting when they turned three years old, and the girls' Isurim come at once if they were spurting when they turned 12. We need not say that they were born the same day, or that they were spurting the entire time. It suffices that they did not immerse in between.
Answer #3 (to Question #3 - Mishneh l'Melech Ishus 2:18 DH v'Hashta): They brought Simanim of Seris and Ailonis (such Simanim appear before adulthood).
Rejection (Aruch l'Ner 34a DH b'Shof'os): If so, they would not need to separate for three months for Havchanah! Perhaps we can say that they separated because they were Safek Serisim and Ailoniyos, but after 20 years it is revealed retroactively that they were Vadai (and were adults from age 12 and 13).
Answer #4 (to Question #3 - Mishneh l'Melech Ishus 4:7 DH Shuv): Rashi holds like R. Tam and the Rosh (Nidah 6:2), that R. Yehudah says that during the Perek (the last day of her 12th year) is like after the Perek. (If hairs were found on that day, the child becomes an adult immediately at night. The Mishneh l'Melech's text of the Rosh omitted 'and a day'. Our texts of the Rosh have 'and a day' in brackets.)
Note: Rashi (Nidah 48b) says that before the Perek is Onas Nedarim (the year before adulthood). This connotes that during the Perek is Onas Nedarim. It seems that Rav could establish the Mishnah to discuss women who were raped (or married and divorced) and gave birth to boys the same day. They accepted Kidushin to take effect on day 41 (right after Dam Tohar ends), and they were spurting blood when day 41 began. However, in this case Levi argues with Rav and says that they do not become Nidah (Nidah 35b). Rabbeinu Meir (brought in Tosfos ha'Rosh 34a DH v'Ha) says that there is Kares after Chupah for Eshes Ish, but before Chupah there is no Kares for Na'arah Me'orasah. We must say that regarding Isur Chal Al Isur, Eshes Ish and Na'arah Me'orasah are considered the same Isur.
Mishneh l'Melech (Hilchos Ishus 2:21): The Sugya in Yevamos (34b) proves that we do not require me'Es l'Es.
Rebuttal (R. Akiva Eiger): According to Tosfos' text there is no proof.
Question: Rashi (Erchin 18a DH me'Es) explains that 'the years for boys and girls' are me'Es l'Es. Yevamos 34a implies that a child becomes Bar or Bas Mitzvah at the start of the day!
Answer (Avi Ezri Ishus 2:21): The years for boys and girls in the Beraisa refers to Nedarim. The Rambam says that if a boy vowed after 13 (or a girl after 12) years me'Es l'Es of hours, we need not check his vow, even if he did not bring Simanim. Letter of the law, the year extends me'Es l'Es of hours. However, part of the day is like the entire day if he brought Simanim; this is the case in Yevamos.
Poskim
Shulchan Aruch (OC 55:9): One is a minor until he brings two hairs after 13 years and one day.
Mishnah Berurah (42): Even one moment of the extra day suffices. If one was born on Rosh Hashanah, even at the end of the day (but surely before night), he becomes Bar Mitzvah on the night of Rosh Hashanah of the 14th year.