Bamidbar 3:15

מבן חדש ומעלה [NUMBER THE CHILDREN OF LEVI … EVERY MALE] FROM A MONTH OLD AND ONWARDS — As soon as it passed the age of a non-viable child (a full month was regarded as the utmost time such a child can live) it can be counted to come under the term “keeper of the holy charge”. — R. Judah the son of R. Shalom said: It is quite the usual thing with this tribe to be included in a census from their very birth, for it is said, (Numbers XXVI 59) “[And the name of Amram’s wife was Jochebed, the daughter of Levi], whom her mother bore to Levi in Egypt” — which is explained to mean that she bore her just as she entered the gates of the border-city of Egypt and yet she is counted among the seventy souls which came down with Jacob into Egypt (cf. Genesis 46:27 and Rashi thereon), because if you count their number (as set forth in Genesis 46:8—27) you will find that they were one short of seventy, and it was she whose name is not mentioned there who made up the number seventy (cf. Midrash Tanchuma, Bamidbar 16).

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.

פְּקֹד֙ אֶת־בְּנֵ֣י לֵוִ֔י לְבֵ֥ית אֲבֹתָ֖ם לְמִשְׁפְּחֹתָ֑ם כׇּל־זָכָ֛ר מִבֶּן־חֹ֥דֶשׁ וָמַ֖עְלָה תִּפְקְדֵֽם׃