Themes and concepts (theology?) present in these verses:
• Purification – “Take the Levites from among the people of Israel and cleanse them. (Numbers 8:6 ESV)
• Consecration – When you bring the Levites before the Lord, the people of Israel shall lay their hands on the Levites, and Aaron shall offer the Levites before the Lord as a wave offering from the people of Israel, that they may do the service of the Lord. (Numbers 8:10, 11 ESV)
• Sanctification – “Thus you shall separate the Levites from among the people of Israel, and the Levites shall be mine. (Numbers 8:14 ESV)
• Replacement – For all the firstborn among the people of Israel are mine, both of man and of beast. On the day that I struck down all the firstborn in the land of Egypt I consecrated them for myself, and I have taken the Levites instead of all the firstborn among the people of Israel. (Numbers 8:17, 18 ESV)
• Mediation – And I have given the Levites as a gift to Aaron and his sons from among the people of Israel, to do the service for the people of Israel at the tent of meeting and to make atonement for the people of Israel, that there may be no plague among the people of Israel when the people of Israel come near the sanctuary.” (Numbers 8:19 ESV)
• Obedience – Thus did Moses and Aaron and all the congregation of the people of Israel to the Levites. According to all that the Lord commanded Moses concerning the Levites, the people of Israel did to them. (Numbers 8:20 ESV)
The firstborn of the earth belong to God, because the earth received from God the firstborn of Heaven.
Effective working age for Levites is 25 to 50. Might be important.