One month & upward – B’midbar
In the census, the members of the tribe of Levi were counted from the age of one month (Num. 3:14), though Ramban points out the other tribes were counted from the age of 20.
The difference is because the males from the other tribes were needed for military service, from which the Levites were exempt.
From the law of the Levites we learn the reason why the rule of pidyon ha-ben, the redemption of the first-born son, applies when the child is a month old (Ex. 13:2,13; Num. 18:15).
Rashi tells us that a child’s viability is in doubt up till then, but from the age of one month it is assumed that his life is stable and he will survive.
The rabbis look at the end of life too and say that a person is deemed to be alive up to his last moment. Once he has died it doesn’t matter how famous he was, even King David himself.
What is the criterion? Whether he can observe the commandments. What gives a person status is not his possessions – “When he dies he takes nothing with him” (Psalm 49:18) – or how notable he was.