Contributed by: R. Ezra Mizrahi
"Count the sons of Levi according to their fathers' household, according to ther families, every male from one month of age and up shall you count them." (3:15)
Rashi explains here that from after 30 days, there is much less of a chance of the male dying in infancy. Specifically, Rashi says, "As soon as it passed the age of a non-viable child (a full month was regarded as the utmost time such a child could live), the child could be counted."
Later in this very parasha, we find the commandment to count our first born sons: "Hashem said to Moses, 'Count every firstborn male of the children of Israel from one month of age and up, and take a census of their names."
Interestingly, Rashi here changes his words: "as soon as he has passed the age when he might possibly (safek) prove non-viable."
We see also in Baba Kama, that the Torah commanded us to redeem our first born at the age of one month. Rashi says there as well that from that time and on, the child is out of "safek nefilim".
Why is it that Rashi changes his words with regard to counting Bnei Yisrael versus counting the Levi'im?
The difference is that by the Levi'im, the counting was done "Al Pi Hashem", by the word of God. Moshe would walk to each tent, wait outside, and a divine voice would be heard, specifying the number of people in the tent. Therefore in this case, there is no need for Rashi to say "safek" or doubt of being 30 days old or not, for Hashem was doing the counting, and He has no doubt.