1 Chronicles — Chapter 23

Aaron was set apart, that he should minister in the Holy of Holies, he and his sons for ever, and burn incense before the Lord, according to his rite, and bless his name for ever. Lapide: God chose Aaron and his firstborn sons as high priests; the other sons became lesser priests. Only the high priest entered the Holy of Holies; other priests burned incense at the altar of incense before the Holy of Holies.

But the sons of Moses the man of God were numbered in the tribe of Levi. Lapide: Admirable was Moses's resignation and humility, by which he submitted to God's ordinance — which transferred the primacy to Josue and numbered his sons not among the priests but among the Levites. For the priesthood by God's decree was given to the posterity of Aaron, Moses's elder brother.

These are the sons of Levi by their families, heads of families, who were numbered... from twenty years old and upward, who did the work of the ministry of the house of the Lord. Lapide: David changed the age of Levitical service from thirty years (as Moses had set, for the burden of carrying the Tabernacle required robust strength) to twenty years — since now that the Temple was built and Israel was at rest, the reason for the higher age had ceased.