QUESTION: The Mishnah states that a father may accept an oath of Nezirus on behalf of his son.

What significance is there to the Nezirus of a minor (Katan)? Since the Torah does not obligate a Katan to observe the Mitzvos, why does the Torah require that he observe the Nezirus which his father accepted on his behalf?

(a) The RAMBAM (Hilchos Nezirus 2:13) explains that the obligations of this Nezirus apply to the father. The father must treat his son like a Nazir by keeping him away from wine, haircuts, and Tum'ah. This also appears to be the intention of the ROSH who writes that the father must bring all of the Korbanos for the Nezirus of his son.

(b) RASHI in Sotah (23a, DH ha'Ish; see also BARTENURA to Sotah 3:8) writes that when the father accepts Nezirus for his son, his son remains a Nazir even if he reaches adulthood during the period of the Nezirus. Rashi's intention may be to explain the relevance of the Nezirus to the son. Rashi explains that even the child himself, and not just the father, is affected by the Nezirus because -- if he becomes an adult and becomes obligated in Mitzvos during the Nezirus -- he will become obligated mid'Oraisa to observe all of the laws of Nezirus as a result of the Nezirus which took effect while he was a Katan. (Although Mitzvos do not apply to a Katan, a change of status, like the status of Nazir, does apply to a Katan, such as the status of Tum'ah and Taharah.)

RASHI in Makos (22a, DH Nazir Shimshon) and the MEFARESH (Nazir 30a, DH Ka'im, and DH Hachi Garsinan) also imply that if the father makes his son a Nazir, the Nezirus remains in force even after the son becomes an adult.

However, TOSFOS here (DH v'Ein), the ROSH, and other Rishonim cite a Tosefta (3:9) which clearly states that just as Macha'ah (protest) ends the son's Nezirus and prevents him from bringing the Korbanos of a Nazir, so, too, adulthood (the growth of two Se'aros) ends his Nezirus and prevents him from bringing the Korbanos of a Nazir. This is also clear from the Gemara later (beginning of 30a) which, according to these Rishonim, bases its question on the fact that once the son reaches adulthood, the Nezirus which his father accepted for him is no longer valid (and he must make himself a Nazir if he wants to observe a full Nezirus).

How does Rashi understand the Tosefta? Perhaps Rashi maintains that when a son reaches adulthood after his father made him a Nazir, although he must observe his Nezirus he does not bring the Korbanos at the conclusion of the Nezirus. This is because the Korbanos are part of the obligation which a person accepts when he makes an oath of Nezirus (see Nedarim 4a). Since the father accepted the obligation of the Nezirus on behalf of his son, it becomes the father's obligation to bring Korbanos. When his son reaches adulthood (and is no longer in his father's domain), he does not acquire the obligation of Korbanos from his father, but the father is also not obligated to bring the Korbanos because his son has left his domain. The Tosefta means only that the Nezirus of the son is no longer applicable with regard to its Korbanos. Therefore, the laws of the Mishnah apply to a son who reaches adulthood after his father made him a Nazir (for example, if his father set aside for him an animal before he became an adult, it is a Chatas Mesah).

The Gemara later (beginning of 30a) also might mean that since the son does not bring Korbanos when he reaches adulthood, he is not bound by all of the laws of Nezirus. (This also answers the questions of the KEREN ORAH on the Mefaresh there.)

Accordingly, it is clear why the Mishnah -- which mentions Macha'ah as a way of annulling the Nezirus initiated by the father -- does not mention that adulthood also annuls the Nezirus. The two Halachos differ significantly: Macha'ah halts the observance of the Nezirus entirely, while adulthood merely prevents the son from bringing the Korbanos of Nezirus.