You are to take the Levites for Me—I am the LORD—in place of all the firstborn of Israel, and the livestock of the Levites in place of all the firstborn of the livestock of the Israelites.”
Here is the exchange made of the Levites for the first-born. 1. The first-born were numbered from a month old, Num 3:42, Num 3:43. Those certainly were not reckoned who, though first-born, had become heads of families themselves, but those only that were under age; and the learned bishop Patrick is decidedly of opinion that none were numbered but those only that were born since...
Commenting on Numbers 3:40-51
And Moses numbered, as the Lord commanded him,.... No doubt assisted by others, though not mentioned: all the firstborn among the children of Israel; which some think were only those that were born since they came out of Egypt, as Bonfrerius, Bishop Patrick, and others.
the cattle of the Levites--These, which they kept to graze on the glebes and meadows in the suburbs of their cities, to supply their families with dairy produce and animal food, were also taken as an equivalent for all the firstlings of the cattle which the Israelites at that time possessed. In consequence of this exchange the firstlings were not brought then, as afterwards, to the altar and the priests.