From above the Ark of the Covenant, God instructed Moses concerning the sanctification of the tribe of Levi. Their purpose, practical function and spiritual position had already been described and established; it was time to insert them into their proper office. God ordained that specific sacrifices be made in order to commence the service of the tribe.
After the light of the candlestick was lit, Moses was to sanctify and dedicate the Levites in preparation for their ministry. The prescribed ritual went like this: they were supposed to shave their bodies, wash their clothes and be anointed with holy water. Then there were several offerings that had to be offered.
A meat offering, a sin offering and a burnt offering accompanied the offering of the Levites themselves to God. The tribe of Levi became a living sacrifice to God. God claimed them for Himself. According to Him, He had a right to them because He had spared the firstborn Jews while He killed all of the eldest offspring in Egypt. Of course, God being who He is... and, God doing things the way He customarily does them... He gave the tribe to Aaron and to his sons (Numbers 8:19). He gave! God is a giver! He gave what (or who) He took on the same day that He had received them.
Since the other tribes were not allowed to come close enough to God to assist the priests, the Levites would do it for them. Each Levite would have a 25 year ministerial carrier ... including (it seems) the first five years which were probably set aside for training and preparation (compare Numbers 8:24 to Numbers 4:3, 23, 30, 35 & 39). All of these ministers were to be forced into retirement at the age of 50. Well, semiretirement would be a better description. They still had access to the tabernacle complex and were servants to their fulltime brothers who were still young enough for the primary tasks.