“An ox or a lamb… and from the eighth day and onwards it shall be accepted as an offering!” (Vayikra 22:27)
It is appositive commandment that every animal offering should be at least eight days old, as it says “An ox, lamb or goat which is born should be eight days with its mother and from the eighth day and onwards it shall be accepted as an offering!”, and the Torah teaches us here that before the eighth day it cannot be accepted as an offering. This type of deduction is called a negative commandment which comes from a positive commandment, and has all the rules of a positive commandment.
Amongst the roots of this commandment are that which we have already explained that a person is awakened to correct his ways by performing certain actions, and we have therefore been commanded that the performance of those actions which cause the rectification of his ways to be in the ultimate state of perfection. Part of this state of perfection is that the offering should be at least eight days old, for before this age it is unfit for any purpose and a person would not want it for eating, sale or a present.
This commandment applies when the temple was standing, to priests, as the commandment of bringing the offering is incumbent upon them, and it is through them that the offering is brought. According to the Rambam the obligation of this commandment is even for the members of Klal Yisrael who sanctify the offering, and according to his opinion the obligation is for priests and laymen, to men and to women.