The Torah in Parashat Bamidbar refers to the tribe of Levi as “shomerim mishmeret ha-kodesh” (“watchmen of the sacred guard” – 3:38), and the Gemara (Tamid 26a) cites this verse as a source for the mitzva of shemirat ha-Mikdash – requiring stationing guards from the tribe of Levi around the Temple. As the Mishnayot discuss in the beginning of Masekhet Middot, kohanim would stand guard at various points inside the Beit Ha-mikdash, and Leviyim would stand guard at various posts in the area outside the Temple.
The first Mishna in Masekhet Tamid specifies the different locations inside the Beit Ha-mikdash where kohanim were stationed to stand guard, two of which were guarded by the “rovim” – young kohanim. The mefaresh (the anonymous commentary to Masekhet Tamid) explains that these were kohanim who had not yet reached the age of mitzva obligation – meaning, they were below the age of thirteen – and were thus unable to serve in any other capacity. Since those above the age of thirteen preferred serving in the other priestly roles, these young kohanim were assigned as guards.
Later writers questioned how it was possible for kohanim below the age of mitzva obligation to fill this role which is mandated by the Torah. Seemingly, just as in all other areas of Torah law, halakhic obligations can be fulfilled only by those who have reached the age of mitzva obligation. Indeed, the Mishneh Le-melekh (Hilkhot Beit Ha-bechira 8:5) disagrees with the mefaresh’s interpretation, and asserts that the Mishna refers to teenage kohanim. The Rambam writes that although kohanim are qualified to perform the service in the Beit Ha-mikdash from the age of thirteen, in practice they did not begin until the age of twenty. It thus stands to reason that when the Mishna speaks of youngsters serving the role of guards, it refers to the teenage kohanim who were technically qualified to serve in the Beit Ha-mikdash but in practice did not perform ritualistic functions, and instead were given the role of guarding the Mikdash.
Others, however, suggest defending the claim of the mefaresh that this role was served by kohanim below the age of bar-mitzva. The Minchat Chinukh (388:2) asserts that this mitzva is an obligation cast upon the nation at large, who are to see to it that the Temple is guarded by members of the Levite tribe. The kohanim who guard are not the ones who fulfill the mitzva; rather, the obligation is fulfilled collectively by Am Yisrael, whose leadership ensures that the Mikdash is guarded as the Torah commanded. As such, those who guard do not need to be adults who have reached the age of Torah obligation. For that matter, the Minchat Chinukh writes, a cheresh (deaf-mute) and shoteh (a person with mental dysfunction) are likewise suitable for fulfilling this role, despite being exempt from Torah obligation. The Minchat Chinukh draws a comparison to the consumption of sacrifices, which is a mitzva cast collectively upon the kohanim, as noted by the Rambam (Hilkhot Ma’aseh Ha-korbanot 10:1). Later (10:17), the Rambam writes that even young children of kohanim are permitted to eat the priestly portions of sacrifices. This would seem to prove that mitzvot assigned collectively to the kohanim may be fulfilled even by those under the age of bar-mitzva, and this would also be the case with regard to the obligation to guard the Temple.
Rav Avraham Borenstein of Sochatchov, in his Avnei Neizer (Y.D. 449), dismisses this comparison drawn by the Minchat Chinukh between guarding the Mikdash and the consumption of sacrifices. He writes that children are of course capable performing an act of eating, and thus their consumption of sacrifices suffices to fulfill the mitzva that the sacrifices be consumed. Other mitzva acts, however, are halakhically meaningful only if they are performed by people obligated in the mitzva.
We might respond that the Minchat Chinukh perhaps adopted a minimalist view of the obligation of shemirat ha-mikdash, perceiving it as requiring nothing more than the physical presence of kohanim at various spots in the area of the Beit Ha-mikdash. As such, there is no difference whatsoever between standing guard and eating food in this regard; in both instances, the act is being done, and thus the obligation is fulfilled. The Avnei Neizer perhaps assumed that since the guarding was done out of respect and honor for the Beit Ha-mikdash, it requires not just the physical presence of people, but a certain level of cognizance and awareness, which only a person who has reached the age of bar-mitzva is capable of. And for this reason, perhaps, in the view of the Avnei Neizer, the law allowing minors to partake of sacrifices does not serve as a valid precedent for allowing minors to guard the Mikdash.