Numbers 8:5

And the Lord spake with Mosheh, saying:

Now after the firstborn and the Levites had been counted, and the Levites were commanded about their [individual] work and charges, and he gave them the wagons,, 7:5-8. He completed the commandments referring to the priests with the [laws of the] candelabrum; then He came to give the commandments concerning the Levites so that they could begin their service, and therefore it was necessary to purify them and to effect atonement for them. This is the reason why this chapter [of the purification of the Levites] is put in juxtaposition to this [section on the candelabrum]. He finished [the laws of the Levites] with a commandment applicable for all generations: From twenty and five years old and upward ‘yavo litzvo tzava’ (they shall come to be counted among the host) for the work of the Tent of Meeting.
Now Rashi commented: “Here it says, from twenty and five years old; but in another place it is said, from thirty years old! How are these [contradictory verses to be reconciled]? From twenty-five years of age the Levite comes to learn the laws of the service, and he studies for five years, and at the age of thirty he performs the service. From here [we derive the principle] that if a pupil studying under his master does not see progress in his study after five years, he will never see such progress.” If this is so, the expression yavo litzvo tzava means that “he shall come and assemble among those who are gathered” for the work of the Tent of Meeting, meaning that he is to stay with them and continually observe the service [of the older Levites] by day and night, in order to learn the laws of the service in theory and practice. This is why He did not state here “to do service,” as He said above [From thirty years old and upward … ‘to do’ service],, Verse 23. For since Scripture speaks there of the Levites above the age of thirty years, it uses the expression ‘la’avod avodah’ (to do service). Here, however, it refers to the five-year period of their training; hence the expression ‘ba’avodath’ (in the service), as they were not yet permitted to do the actual service. but He stated, ‘yavo litzvo tzava’ (they shall come to be counted among the host) ‘ba’avodath’ (for the work of) the Tent of Meeting, meaning that he is to come among those who are gathered for the purpose of the service. But I do not know whether this [explanation that …