Sifrei Bamidbar 62

(Bamidbar 3:24) "This is what applies to the Levites. From the age of twenty-five, etc." "Years" (i.e., a deficiency in years) disqualify them (from Levitical service), but blemishes do not. For it would follow (otherwise), viz.: If where years do not disqualify (i.e., with the Cohanim), blemishes do disqualify, then where years do disqualify, how much more so should blemishes disqualify! It is, therefore, written "This is what applies to the Levites, etc." "Years" disqualify the Levites, but not blemishes. (Bamidbar 8:24) "From the age of twenty-five and up, etc.": One verse states "From the age of twenty-five and up," and another (Ibid. 4:23) "From thirty years and up." How are these two verses to be reconciled? From the age of twenty-five for learning (the Levitical service), and from the age of thirty, for serving.