Midrash su Numeri 8:24

(Fol. 24) One passage says (Num. 8, 24) From twenty and five years old and upward, and again another passage says (Ib. 4, 3) From thirty years old; how can both passages be reconciled? The age of twenty-five years refers to the time of studying and the thirty years of age refers to actual work. From this it may be inferred that a scholar who does not see a sign of improvement for a period of five years, will hence never see an improvement. R. Jose says: "If he does not see for a period of three years, as it is said (Dan. 1, 5) And that they should be nourished three years; and that he should teach them the learning and the tongue of the Chaldeans." The former Tanna, however, contends that three years were sufficient for the Chaldaic language, which is easy to learn. The latter Tanna holds that only concerning the rules of the Temple work, which were so numerous a period of five years was necessary.

(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.