From a month old, because at that time the first-born, in whose stead the Levites came, Numbers 8:16, were offered to God, Luke 2:22, and to be redeemed, Numbers 18:16. And from that time the Levites were consecrated to God, and were as soon as they were capable, to be instructed in their work. Elsewhere they are numbered from twenty-five years old, when they were entered as novices to part of their work, Numbers 8:24; and from thirty years old, when they were completely 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.