We can realize why God's mandate (at 8 days old), because the child can suffer a hemorrhage between the first 24 hours of life and the seventh day of life (this bleeding is called early VKDB or VKDB classic) since every child is born with vitamin K deficiency which is necessary for blood clotting and during circumcision the risk of hemorrhage increases.
It is significant to mention that the classical VKDB is the most common and is related to delayed or INADEQUATE feeding. At present there is a theory that has not yet been medically deepened, which states that the mother should take a daily supplement rich in vitamin k after childbirth TO INCREASE THE INTAKE OF VITAMIN K IN BABIES through breast milk; Now let's review the diet in the old testament of the people of God, they were FOODS RICH IN VITAMIN K (DRIED VEGETABLES, CHICKPEAS, LENTILS, BEANS, etc.) which the mother could impart to the baby.
Brought beds, and basins, and earthen vessels, and wheat, and barley, and flour, and parched corn, and beans, and lentils, and parched pulse, and honey, and butter, and sheep, and cheese of kine, for David, and for the people that were with him, to eat: for they said, The people is hungry, and weary, and thirsty, in the wilderness. (2 Samuel 17: 28-29)
Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things. But flesh with the life thereof, which is the blood thereof, shall ye not eat. (Genesis 9: 3-4)
It is amazing how our God is very prolific in his design and in his ordinances; before giving the mandate of the circumcision (8 days old) had anticipated many years ago another ordinance on diet in vegetables.
The word of God (Bible) is infallible and anticipates scientific studies revealing a truth in advance.
It should be noted that 3400 years ago it was not known, nor understood the circulation of blood, its components, its coagulation and the functions of vitamins.
No doubt its design with its ordinances are perfect and consistent up to the smallest detail, admirable, right.