Matthew Poole and continuators on 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.

Because at that time the first-born, in whose stead the Levites came, Numb. 8. 16. were offered to God, Luk. 2. 22. and to be redeemed, Numb. 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 numbred from twenty five years old, when they were entred as novices to part of their work, Numb. 8. 24. and from thirty years old, when they were compleatly admitted to their whole office.