"And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.
Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them."
What appears to be happening in this chapter is the Lord set aside the Levites to administer service in the priesthood. Instead of the firstborn of every household being given. I'm sure that was much easier to say your of this lineage this is where you serve. The firstborn for all the tribes were counted and their number was greater than the number of Levites. So redemption money was paid to Aaron to redeem those not serving. So now all is equal.
What I sat and pondered on is why count a male child a month old and upward? What service could a one month old give being called to serve in the priesthood?
This made me think of John the Baptist and when he was ordained to the priesthood.
"For he was baptized while he was yet in his childhood, and was ordained by the angel of God at the time he was eight days old unto this power, to overthrow the kingdom of the Jews, and to make straight the way of the Lord before the face of his people, to prepare them for the coming of the Lord, in whose hand is given all power." (Doctrine & Covenants 84:28)
I guess there is no age requirement on when the Lord can call someone. I might think of a deacon being 12 years old, but truly it can be totally up to the Lord when he calls them. In reality they were all called to this before they were even born.
This sent me on a scripture chase and I read about foreordination. It is spoken of in every set of scriptures.
"Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations." (Jeremiah 1:5)
"Who hath saved us, and called us with a holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began," (2 Timothy 1:9)
"And this is the manner after which they were ordained - being called and prepared from the foundation of the world according to the foreknowledge of God, on account of their exceeding faith and good works; in the first place being left to choose good or evil; therefore they having chosen good, and exercising exceeding great faith, are called with a holy calling yea, with that holy calling which was prepared with, and according to, a preparatory redemption for such." (Alma 13:3)
"And God saw these souls that they were good, and he stood in the midst of them, and he said: These I will make my rulers; for he stood among those that were spirits, and he saw that they were good; and he said unto me: Abraham, thou art one of them; thou wast chosen before thou wast born." (Abraham 3:23)