What does the word "Zos" come to preclude?
Rashi and Targum Yonasan: It comes to preclude Mumin, which do not disqualify a Levi from the Avodah.
How will we reconcile this Pasuk, which gives the age of the commencement of the Levi'ims' Avodah, as twenty-five, with the Pasuk in Bamidbar, 4:3, which gives it as thirty?
Rashi: Whereas the Pasuk in Bamidbar is referring to the age at which the Levi'im actually begin serving, the current Pasuk is referring to the age at which he begins his apprenticeship. 1
Ramban #1: Moshe and Aharon appointed the Levi'im from the age of thirty. However, any ben Levi from the age of twenty-five who felt that he was capable of serving, was welcome to come and assist the older Levi'im in their work. 2
Rashbam: The Pasuk in Bamidbar, and the Pasuk at the beginning of Naso, is referring to the Avodah of carrying between the ages of thirty and fifty; whereas the current Pasuk is referring to the Avodah of guarding, 3 which the Levi'im are obligated to perform five years earlier