Why were the Levi'im counted only from 30 days and upwards, and not from birth?
Rashi: Because the 30 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 firstborns, who could only be redeemed after 30 days, 2 they too, were counted from 30 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 (26:59, about the birth of Yocheved) "Asher Yaldah Osah l'Levi b'Mizrayim" (Levi's wife bore her to Levi in Egypt) - as they entered Egypt. And she completed the count of 70 people who came down to Egypt (See Rashi).
Why does it not mention Aharon here, like it does later (3:59)?