Numbers 18:16

And as to his redemption money, from a month, &c. The pronoun -his" refers to the firstborn of men, who were redeemed from death by 5 shekels. The price for the firstborn of unclean animals varied according to the priests" valuation (Leviticus 27:11 f., Leviticus 27:27).

from a month old i.e. directly the age of a month has been reached, and so it practically means - ata month old."

the shekel of the sanctuary See on Numbers 3:47.