From a month old and upward... — The males of the other tribes had been numbered “from twenty years old and upward” (Numbers 1:3). The firstborn males, however, among all the children of Israel, in whose place the Levites were taken, wer-directed to be numbered “from a month old and upward” (Numbers 3:40; Numbers 3:43); and this was the age afterwards fixed for their redemption (Numbers 18:16).
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.