Gemara

Nazir 29b (Beraisa - Rebbi): A man can impose Nezirus on his son until the son brings two hairs (of maturity);

R. Yosi b'Rebbi Yehudah says, he may do so until the son reaches Onas ha'Nedarim (the age at which his vows can take effect.)

Suggestion: Rebbi holds that imposing Nezirus on one's son is a tradition from Sinai, and applies even from Onas ha'Nedarim, until (adulthood, i.e. 13 full years and) two hairs. R. Yosi b'Rebbi Yehudah says that imposing Nezirus on a son is for Chinuch (training to do Mitzvos). After Onas ha'Nedarim, the son (can vow by himself, so he) leaves the Reshus of the father.

Rejection #1: Both agree that it is a tradition. They argue about the vows of a child at Onas ha'Nedarim. Rebbi holds that his vows are only mid'Rabanan. The tradition transcends it (it enables the father to impose Nezirus mid'Oraisa). R. Yosi b'Rebbi Yehudah holds that his vows are mid'Oraisa.

Rejection #2: Both agree that it is for Chinuch, and the vows of a near-adult are mid'Rabanan. Rebbi holds that Chinuch, mid'Rabanan, transcends the child's own ability to vow mid'Rabanan. R. Yosi b'Rebbi Yehudah says that once the child can vow by himself, there is no Mitzvah for the father to train him in Nezirus.

The following Tana'im argue as Rebbi and R. Yosi b'Rebbi Yehudah do:

(Beraisa): R. Chanina's father imposed Nezirus on him. R. Gamliel was checking Chanina for signs of adulthood. (This would nullified the Nezirus);

R. Yosi says, he was checking whether or not Chanina understood vows.

Chanina: Don't bother! If I am still a child, I am a Nazir due to my father. If I am mature, I accept Nezirus upon myself!

Question: According to Rebbi, even if he was mature (reached Onas ha'Nedarim, but not adulthood), he is a Nazir due to his father!

Answer: Rather, Chanina said 'I am a Nazir due to my father, or due to myself' (and stipulated):

If I brought two hairs from the beginning (before my father made me a Nazir), I am a Nazir due to my acceptance. If I will bring two hairs at the end (after finishing Nezirus), I was a Nazir due to my father.

Nidah 45b (Mishnah): If a girl above 11 (full) years vows, we check her (to see if she understands that Nedarim are to Hash-m). If a girl above 12 vows, it is valid (we do not check her). During her 12th year we check her. (Every law applies similarly to a boy a year older.)

If she vows before (11 full years), even if she knows (that vows are to Hash-m), it does not take effect. If she vows after (12 full years), even if she says that she did not know, it takes effect.

Question: The Mishnah taught that we do not check her vows after 12 years. Why must it teach that during her 12th year we check? We can infer this!

Answer: Since 30 days in a year are considered like a year, one might have thought that if she did not know in the first 30 days of year 12, we need not check the rest of the year. The Mishnah teaches that this is not so.

(Mishnah): If a boy vows after (year 13), even if he says that he did not know that vows are to Hash-m, it takes effect.

Inference (Rav Hamnuna): The (13th) year is like the year before. (If he brings two hairs, they are Shuma (not a sign of adulthood) regarding punishment.)

Rav Hamnuna infers from the Seifa itself. It cannot be that he did not grow hairs, for if so he is still a minor! Rather, he grew hairs. The hairs are significant after the year. During the year they would be Shuma!

(Beraisa): "Ish Ki Yafli Lindor Neder" includes a boy above 13. Even though he does not know to Whom he vowed, it takes effect.

Question: If he did not grow hairs, he is still a minor!

Answer (R. Zeira): Rather, he grew hairs. The hairs are significant after 13, during year 13 they would be Shuma!

Rishonim

Rambam (Hilchos Nedarim 11:1): If a boy or girl at least a day past 12 or 11 years, respectively, swore or made a vow of Isur or Hekdesh, we check him (or her). If he knows to Whom he vowed or swore, it is valid. If not, it is void. He must be checked the entire 13th year (or 12th for a girl).

Rambam (2): If he vowed or was Makdish at the beginning of the year, and we asked him and found that he knows and his vow was upheld, and he vowed again, even at the end of the year, he must be checked before we uphold it. We do not say that since he knew at the beginning of the year, we need not check him now. Rather, we check the entire year.

Question (Kesef Mishneh): What is the source for this? The Gemara said that one might have thought that if she did not know in the first 30 days of year 12, we need not check the rest of the year, so the Mishnah refutes this. This implies that if she did know at the beginning, we need not check the rest of the year! Perhaps the Rambam's text suggested that if she did know at the beginning, we need not check the rest of the year, and the Mishnah refutes this. This requires investigation.

Radvaz: The Rambam's text is more reasonable than ours. Why should we assume that if she did not know at the beginning of the year, she will not know the entire year?!

Note: We do find such things. A fetus can develop into a viable baby in seven months. If not, it needs nine months to develop. If it is born in the eighth month, it is a Nefel. A Kesav Yad of Perush ha'Mishnayos supports the Kesef Mishneh's answer; the standard text of Perush ha'Mishnayos is like our Gemara (Hagahos Tur ha'Shalem 6).

Rambam (3): Before his 12th (and her 11th) year, even if he knows to Whom he vowed, it is not a vow. After 13 (or 12) years and a day, even if he says that he does not know to Whom he vowed, his vow or Hekdesh is valid, even if he did not bring two hairs. This is called Onas ha'Nedarim.

Rebuttal (Mishneh l'Melech): The Gemara (Nidah 45b) explicitly says that after 13, his vows are unconditionally valid (i.e. even if he does not know to Whom he vowed) only if he brought hairs!

Defense #1 (Simlas Binyamin YD 233:1): R. Zeira's opinion is not difficult for the Rambam, who holds that Mufla Samuch l'Ish is mid'Oraisa. He can say that R. Zeira holds that it is mid'Rabanan. However, Rav Hamnuna's inference is difficult. The Rambam unconditionally upholds vows after 13, even if he did not bring hairs! The Rambam holds that Mufla Samuch l'Ish includes two cases: a child whose intellect is close to maturity (a year before adulthood), and one whose years are close (13 years without hairs). This is reasonable only if it is mid'Oraisa. If it is mid'Rabanan, surely they enacted only for one who is 13 and knows to Whom he vowed. The Sugya holds like the opinion that Mufla Samuch l'Ish is mid'Rabanan.

Defense #2 (Shev Shematsa 5:11, b'Sof): This Sugya holds that hairs during the year before adulthood are a sign of adulthood, and "Ish" regarding vows depends on being ready to bring hairs. The Rambam rules that hairs during this year are Shuma; "Ish" depends on years, even without hairs.

Hagahos Maimoniyos (1): Rashi says that Onas ha'Nedarim is the year before adulthood.

Poskim

Shulchan Aruch (YD 233:1): If a boy or girl is at least a day past 12 or 11 years, respectively, and he (or she) knows to Whom he vowed or swore, his vows and oaths are valid, even if he did not bring two hairs. We check him during his 13th (and her 12th) year. Before his 12th (and her 11th) year, even if he knows to Whom he vowed, it is not a vow. From 13 years (for a girl, 12 years) and a day and onwards, even if he does not know to whom he vowed, his vow is valid.

Shach (2): This is even if he did not bring two hairs.

R. Akiva Eiger (DH Af): We consider the vow valid even if we do not see hairs, for we are concerned lest they came and fell out (EH 155:2).

Question: Why did the Gemara ask 'if he did not grow hairs, he is still a minor'? Perhaps after year 13 we are concerned lest hairs came and fell out!

Answer (R. Akiva Eiger): The Gemara connotes that the vow is Vadai valid, and an adult would be lashed for eating what the child forbade.