(a) The RAMBAM (Hilchos Nezirus 2:13) explains that the obligations of
Nezirus apply to the *father*. The father must treat his son like a Nazir,
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 Nezirus for his son.
(b) RASHI (Sotah 23a, DH ha'Ish; see also BARTENURA, Sotah, end of Perek 3)
writes that when the father makes his son into a Nazir, the son remains a
Nazir even if he reaches adulthood during the Nezirus. Rashi might be
answering our question by explaining that even the child himself is affected
by the Nezirus and not just the father since, if he becomes an adult and
becomes obligated in Mitzvos, he will become obligated in all of the laws of
Nazir because of the Nezirus that took effect on him while he was a Katan.
(Even though 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 seem to be of the opinion that if the
father makes the son a Nazir, the Nezirus remains even after the son becomes
an adult.
However, TOSFOS here (DH v'Ein) and the ROSH and other Rishonim cite a
Tosefta (3:9) which states clearly that just like Macha'ah ends the son's
Nezirus and prevents him from bringing the Korbanos of a Nazir, so, too,
reaching adulthood (the growth of two Se'aros) ends his Nezirus and prevents
him from bringing the Korbanos of a Nazir. This is clear as well from the
Gemara later (beginning of 30a) which, according to these Rishonim, is
basing its question on the fact that once the son reaches adulthood, the
Nezirus that his father made for him is no longer valid (and he must make
himself a Nazir if he wants to observe a full Nezirus).
How does Rashi explain the Tosefta? It is possible that Rashi maintains that
when a son reaches adulthood after his father made him a Nazir, although he
still must observe his Nezirus, nevertheless he does not bring the Korbanos
at the conclusion of the Nezirus. The reason is because the Korbanos are
part of the obligation that a person accepts when he first accepts Nezirus
upon himself (see Nedarim 4a). Since the father accepted the obligation of
the Nezirus on behalf of his son, it becomes the *father's* obligation of
Korbanos. When his son leaves his domain at adulthood, the son does not
acquire the obligation of Korbanos from his father, and nor does the father
have to bring the Korbanos, because his son is now out of his domain. The
Tosefta means only that the Nezirus of the son is no longer applicable with
regard to bringing Korbanos, and therefore the laws of our Mishnah may be
repeated for a son who reached adulthood after his father made him a Nazir
(that is, if an animal was set aside, it is a Chatas Mesah, etc.).
The Gemara later (beginning of 30a) also might mean to say that the son does
not have the full laws of Nezirus when he reaches adulthood, since he does
not bring Korbanos. (This answers the questions of the KEREN ORAH on the
Mefaresh.)
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 makes annuls the Nezirus. The two Halachos are very
different: Macha'ah stops the entire observance of the Nezirus, while reachi
ng adulthood only prevents the son from bringing the Korbanos of Nezirus.