John Wesley on Numbers 3:15

From a month old - Because at that time the first - born, 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, Num 8:24, and from thirty years old, when they were admitted to their whole office.