Commentary for Numbers 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).

כל וכר מבן חודש ומעלה, every male above the age of one month. The reason the Levites were counted from the age of one month is that they replaced the firstborn who also were viable at the age of one month. The reason the firstborn had to ransom himself was that a Levite was going to take his place and he had to redeem himself from his former obligation, i.e. sanctity, an obligation which was due to his birth as a firstborn. You may well ask why the firstborn Levites nowadays do not have to redeem themselves. The reason is that the Levites who replaced the firstborn at that time had been appointed by G'd to replace the firstborn of all future generations. Their sanctity was of a permanent nature. Perhaps the Torah alludes to this with the repetition of the words in verse 12: הנה לקחתי את הלוים….והיו לי הלוים. "I have taken the Levites….and the Levites shall be Mine." The second word הלוים seemed redundant otherwise.

כל זכר מבן חדש ומעלה תפקדם. Seeing that they needed to redeem the firstborn of the Israelites at large whose redemption must take place when they are one month old, the Levites also had to be counted already from that age onward.