Why were the Levi'im counted only from thirty days and upwards, and not from birth?
Rashi: Because the thirty days after a person is born he is a Safek Nefel (miscarriage). From the moment that he leaves that Safek, however, he falls under the category of 'Shomer Mishares ha'Kodesh". 1
Rashbam: Because since they were coming to redeem the who could only be redeemed after thirty days, 2 they too, were counted from thirty days.
Rashi (citing R; Yehudah b'R. Shalom): The tribe of Levi was already accustomed to being counted from birth, as the Pasuk states later (in Bamidbar 26:59, in connection with the birth of Yocheved) "Asher Yaldah Osah le'Levi be'Mizrayim" (who Osah (Levi's wife) bore to Levi in Egypt) as they entered Egypt. And she made up the seventy people who came down to Egypt (See Rashi).
Why does it not mention Aharon here, like it does later (in chapter 3 Pasuk 59).