Charles Ellicott Commentary Numbers 3:15

"Number the children of Levi by their fathers` houses, by their families: every male from a month old and upward shalt thou number them." — Numbers 3:15 (ASV)

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, were 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).