“And from 50 years of age he (the Levi) should return from the army of workers, and not work anymore. [Rather,] he will serve with his brothers in the Tent of Meeting to guard what needs guarding, [but as for] work, he shall not work.” These verses (Bamidbar 8:25-26) tell us that after the age of 50, the Levi’im no longer “work” in the Mishkan, but they do “serve” there. Which leads to a discussion about which specific tasks they can no longer do, and which they can.
Rashi tells us that the “work” they can no longer do is “carrying on their shoulder,” while locking the gates (an expression used to mean guarding the perimeter, as there were no “gates” in the Mishkan, and there was nothing to “carry by shoulder” after the Temple was built), singing (praises to G-d while the offerings are brought) and loading/unloading the wagons (which carried the Mishkan‘s parts during travel) are all tasks they can do after 50. The Sifre is usually attributed as Rashi’s source, and his commentary on the previous verse, as well as on part of this verse, is clearly from or at least consistent with, the Sifre. However, the last two items on Rashi’s list of things that Levi’im can do after 50 (as well as specifying which work they can no longer do) is not in the Sifre. Instead, besides “locking the gates,” the Sifre says the 50+ year old Levi “returns” (an expression Rashi also uses) “to do the work of the sons of Gersho[n].” There are numerous issues raised regarding Rashi and the Sifre (see Ramban, the commentaries on Rashi and the commentaries on the Sifre), but I would like to focus on just one; why Rashi deviated from the wording of the Sifre. I will point out, though, that Rashi’s “list” more closely resembles the Sifre Zuta, which says those over 50 do “the work of carrying,” while the tasks they “return” to do are being “gatekeepers, watchmen and singers.” With the exception of “loading/unloading the wagons,” this list is the same as Rashi’s. If we assume that Rashi merged the Sifre and the Sifre Zuta (and that the version of the Sifre Zuta we have was not amended to refkect Rashi’s commentary), or chose one over the other, the question still remains why he did so.
[Interestingly, although in Avos (5:21) Rashi explains the source for “50 years old for advice-giving” to be our verses, with “serving his brothers” after 50 meaning giving them advice, he doesn’t mention this “service” here (see Tz’ror Hamor). I would attribute this to Rashi making a point of explaining 8:26 as “serving with his brothers,” i.e. alongside them, and not “serving his brothers” i.e. assisting them. Since the role of an advisor, as important as it is, is one of assistance, not doing the same tasks as, Rashi could not include it here.]
Only one of the three families of Levi’im, K’has, carried anything on their shoulder; the other two were given wagons to help them transport the things they were responsible for (7:7-9, it should be noted that the tasks of all three are referred to as “work,” which our verses say can only be done by Levi’im between the ages of 30 and 50). Some (e.g. Chizkuni) therefore say that Rashi understood the “Levi’im” being described here to refer only to the family of K’has, whose primary task of carrying the Mishkan’s vessels could not be done after reaching the age of 50, at which point they could do all of the tasks that the other two families of Levi’im did. Numerous questions are asked on this, including why the age of service was 30-50 for all three families if only the task of K’has necessitated this age limit (see Mizrachi’s answer; I don’t think we have to categorize any service done after 50 as “voluntary” in order to call the “army of workers” those between the ages of 30 and 50), but the biggest issue for me is limiting the meaning of “Levi’im” to just one family when every textual indication is that it refers to all three families.
Others (e.g. B’er BaSadeh) say that even though the families of Gershon and M’rari had wagons for transport, and therefore didn’t need to carry anything on their shoulders, since they were not prohibited from doing so, and could have if they wanted to, their task of transport is included in the category of “the work of carrying” forbidden to those 50 and older. [The Torah describes it this way as well, see 4:24, 4:27, 4:31-32.] However, this works (pardon the pun) better with Sifre Zuta’s wording than with Rashi, who uses the word “shoulder.” Nevertheless, since had the wagons not been donated they would have “carried their shoulders,” and they were still allowed to, it qualifies as “carrying on their shoulder.” [Netziv, in his commentary on the Sifre, suggests that they “carried on their shoulders” from the Mishkan to the wagons.] Additionally, since according to Rashi those over 50 could help load/unload the wagons, he had to use the expression “carry by shoulder” in order to differentiate between what they couldn’t do (carry things from station to station) and what they could do (lift things onto and off of the wagons). The question remains, though, why Rashi says that those over 50 can help load/unload the wagons despite it being physical labor (and therefore more easily classified as “work”), necessitating clarifying the “work” that they can’t do as “carrying on their shoulder.”
One of the things the over-50 Levi’im are supposed to do is “keep watch,” or “take care of” (8:26), which Rashi (consistent with his commentary on the previous verse) explains as “to camp around the [Mishkan] and to put [it] up and take [it] down when they travel.” The term “mishmeres” (8:26), “what they are responsible for,” appears in the description of what the families of Gershon (4:27-28) and M’rari (4:31-32) did, but not K’has. It is also the term used by the Sifre Zuta (“watchers,” or “caretakers”) for one of the three tasks the Levi’im over 50 could do. How do they “take care of” those things under their “watch”? They are responsible for taking the Mishkan apart, putting the pieces on the wagons, taking them off the wagons at the next station, and reconstructing the Mishkan. [K’has, on the other hand, only transported the Mishkan’s vessels; the Kohanim took care of them until they were wrapped up and ready for transit and then again as soon as they reached the next station.] It would seem that this is why Rashi said the tasks of the families of Gershon and M’rari could be done even by those over 50 (as long as they don’t carry the items “on their shoulder”).
That Lev’im over 50 can serve as “gatekeepers” is mentioned by both the Sifre and the Sifre Zuta, and is implied in the task of “guarding that which they are in charge of” (8:26).
Although the Sifre says that the Levi’im over 50 can do what the family of Gershon (even those 30-50) was assigned to do, Rashi does not. There is a discussion whether the Sifre also means the family of M’rari when it says Gershon (see Mizrachi), or distinguishes between them because the items M’rari had to load onto and off of the wagons were much heavier than those Gershon was responsible for, and was therefore considered “work” that those over 50 couldn’t do (see B’er BaSadeh). Either way, Rashi has it covered. If the Sifre meant both, Rashi includes both by saying “loading and unloading the wagons,” a task done by both. And if the Sifre meant only Gershon, Rashi omitted it because he is of the opinion that the Levi’im over 50 can in fact do the tasks of M’rari.
Which leaves us with “singing,” mentioned by Rashi (and the Sifre Zuta), but omitted by the Sifre. Here too, some (e.g. Nachalas Yaakov) say that the Sifre would agree that those over 50 can sing, but does not need to mention so explicitly. Others (see Ramban) are adamant that according to the Sifre no Levi over 50 could sing in the Mishkan; it wasn’t until the Temple was built that they could sing no matter how old they were (as long as their voice held out). If Rashi thought that the Sifre meant they could sing, we can certainly understand why he felt the need to mention it explicitly (especially if some might understand the Sifre otherwise). If, on the other hand, Rashi thought that the Sifre and Sifre Zuta disagreed (Bamidbar Rabbah 6:9 says explicitly that Levi’im could not sing in the Mishkan after 50), Rashi chose to follow Sifre Zuta. Perhaps he did so for the same reason so many commentators thought it was so obvious that Levi’im over 50 could sing that the Sifre didn’t need to even mention it. Or perhaps, as Keser Kehunah (a commentary on the Sifre) suggests, the Sifre followed its own opinion (on Bamidbar 15:2) that until the nation entered the Promised Land there were no wine libations, and since the Levi’im sang during the wine libations, there was no singing in the desert. (We’ll put aside for now the singing in the Mishkan from the time they crossed the Jordan until the Temple was built.) Rashi, on the other hand (see Kidushin 37b) says that it was only private offerings that were not accompanied by wine libations before they entered the Promised Land. Since the offerings brought on behalf of the community did have them, the Levi’im sang in the desert too, and this could/should therefore be included in the list of tasks those over 50 were allowed to do. Some (e.g. Netziv), based on Arachin 11a, point out that the term “serve” refers to singing, so Rashi including singing in the tasks the over-50 Levi’im could do when they “served with their brothers.” [That Rashi (4:47) says “the work of the work” refers to the singing done with instruments does not contradict this, as (a) it itself is not called “work,” but “work done during other work,” and (b) there is a difference between the vocals, which is called “serving,” and the instrument playing, which might be considered “work” (see Meshech Chuchmuh). I will add that Netziv’s suggestion that “Gershon” includes the singing because this was one of their main tasks is not persuasive, if for no other reason than that his logic applies to gatekeeping too, yet the Sifre mentions gatekeeping separately.]
The bottom line is that Rashi may be based on the Sifre, but he is more likely based on Sifre Zuta (if he had the same version we do), adjusting the wording regarding the kind of lifting not allowed after 50 and what being “watchmen” meant for added clarity. Or, for the reasons outlined above, he rejected some aspects of the Sifre, and explained the verses the way he thought they should be understood, like the Sifre Zuta.