Parashas Bamidbar · Side page (no TT primary unit)
פְּקֹד֙ אֶת־בְּנֵ֣י לֵוִ֔י לְבֵ֥ית אֲבֹתָ֖ם לְמִשְׁפְּחֹתָ֑ם כׇּל־זָכָ֛ר מִבֶּן־חֹ֥דֶשׁ וָמַ֖עְלָה תִּפְקְדֵֽם׃
Record the descendants of Levi by ancestral house and by clan; record every male among them from the age of one month up.
Bamidbar Rabbah — EXTRA.
מבן חדש ומעלה —\ldots שאין הולד נקרא בן קיימא עד שיהא בן חדש.
English: “From a month old and upward” — \ldots for a child is not called viable until it is a month old.
Custom
Chazal link the one-month threshold to the legal definition of a viable child.