Midrasch zu Bamidbar 8:25

(Bamidbar. 8:25) "And from the age of fifty he shall return from the service of the work." I might think, from all work; it is, therefore, written (Ibid.) "and he shall work no more and he shall serve with his brothers in the tent of meeting." We are hereby taught that he returns to the closing of the gates and to the service of the sons of Gershon (in the tent of meeting [viz. Ibid. 3:25-26]). Rebbi says: Since Scripture speaks of serving at the age of twenty-five and not serving at the age of fifty, if I have learned that from the age of twenty-five he performs all of the services, then not serving from the age of fifty would mean not performing all of the services; it is, therefore, written (Ibid. 8:25-26) "And (after the age of fifty), he shall serve no more, and he shall serve with his brothers, etc." I might think (that after fifty he does no work) even in Shiloh and in the Temple; it is, therefore, written (8:25-26) "And from the age of fifty he shall return from the service of the work … and he shall serve his brothers … but work shall he not perform" — From here they said: Before they entered the land, "years" disqualified Levites (from Levitical service), (but not after they entered the land). And when they entered, the Levites were disqualified (from the service of singing) by (a deficiency in) voice. (8:26) "Thus shall you do with the Levites in their watches": "Years" disqualify in Levites, but not in Cohanim. For it would follow (otherwise), viz.: If where blemishes do not disqualify, years do disqualify, then Cohanim, where blemishes do disqualify, how much more so should years disqualify. It is, therefore, written: "Thus shall you do with the Levites" — and not with the Cohanim. Levites, from the age of thirty until fifty are fit (for service), and Cohanim (are fit from the time they show two (pubic) hairs and thereafter.