Numbers 8:25

And shall no longer work. At the task of carrying on the shoulder, but he returns to lock the gates, to sing, and to load the wagons. This is [the meaning of] “But he shall serve his brethren,” ‘with his brethren’, as Targum [Onkelos] renders.

and from fifty years of age he shall return from the band of the service, and serve no more.

AND FROM THE AGE OF FIFTY YEARS HE SHALL RETURN FROM THE SERVICE OF THE WORK, AND HE SHALL SERVE NO MORE — “the service of carrying loads upon the shoulders, but he returns [i.e., he may still serve] to close the gates, or to sing, or to load the wagons.” This is Rashi’s language. But in the Sifre 63. the Rabbis mentioned only that “he returns to close the gates or to the work of the sons of Gershon” [but they did not say that he may return to the service of singing]! And this indeed appears correct [that a Levite above the age of fifty was not allowed to take part in the singing] — for if it were as the Rabbi [Rashi] has written, why were the twenty-year old Levites not appointed to sing, to close the gates and to load the wagons until they reached the required age, and why were they counted from thirty to fifty years old and only for the work of bearing burdens [since both before and after these two age-limits they could still serve by singing etc., therefore they should have been counted from twenty years, with no upper limit]? Moreover, why were the sons of Gershon and Merari counted in this way [from thirty to fifty] since all their work [i.e., singing, closing the gates and loading the wagons] could be done when they were older?
We must rather say that since they were appointed to carry the ark at the age from thirty to fifty, they [therefore] were not appointed to sing, which was the main function of the Levites, unless they were also qualified to carry the ark, for all those who were appointed to sing were qualified to perform all the services [including the carrying of the ark]. And since the Kohathites [who were the only ones permitted to carry the ark] were counted from thirty to fifty years old, even for singing, they were all counted in this manner, in order that the sons of Gershon and Merari should not be qualified to sing during these years [i.e., below the age of thirty and above the age of fifty] and the sons of Kohath be disqualified. [Hence the age limits of thirty and fifty applied to the Kohathites’ singing, which was their main function, as well as to their carrying of the parts of the Tabernacle, and therefore these age limits applied to the function of singing of the sons of Gershon and Merari as well, and they could not, as Rashi wrote, return to the singing.] But for closing the gates or loading the wagons all of them were qualified [even after the age of fifty, as quoted in the Sifre above]. Furthermore it …

ומבן חמשים שנה ישוב מצבא העבודה של משא, as soon as he has attained the age of fifty he will retire from the duties of carrying parts of the Tabernacle.

The words ולא יעבוד עוד בצבא are to be understood as referring both to the ones younger than 30 and the ones above the age of 50, i.e. both groups not having to performing the physically strenuous type of service. What then were they to do with their time? They were to render service in the form of guard duty, i.e. בשמרותם. The words ככה תעשה ללוים במשמרותם are to be understood as referring to the “training” period of 5 years between their reaching the age of 25 and their attaining the age of 30 when they would assume “active” duties.