From a month old Because at that time the firstborn, in whose stead the Levites came, were offered to God. And from that time the Levites were consecrated to God, and were, as soon as capable, instructed in their work. Elsewhere they are numbered from twenty-five years old, when they were entered as novices into part of their work, (Numbers 8:24,) and from thirty years old, when they were admitted to their whole office.
Related Commentaries of Numbers 3:15
Numbers 3:15
15 Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.