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 Leviyim's Avodah as 25, with the Pasuk in , 4:3, which gives it as 30?
Rashi: Whereas the Pasuk in Bamidbar is referring to the age at which the Leviyim actually begin serving, the current Pasuk is referring to the age at which he begins his apprenticeship. 1
Ramban: Moshe and Aharon appointed the Leviyim from the age of 30. However, any ben Levi from the age of 25 who felt that he was capable of serving, was welcome to come and assist the older Leviyim 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 30 and 50; whereas the current Pasuk is referring to the Avodah of guarding, 3 which the Leviyim are obligated to perform five years earlier - when he is not yet sufficiently strong to carry, and after he has retired from carrying, when he remains obligated to guard the Mishkan. 4
Refer also to 4:3:2:1*. The Ramban explains that this why the Torah writes here "Yavo Litzvo Tzava ... ", implying that the ben Levi should come with those who assemble to perform the Avodah, and not "Yavo La'avod Avodah" - though he (the Ramban) himself considers Rashi's explanation an individual opinion.
Ramban: He was not however, placed in charge of any specific task before the age of 30. See Ramban DH 've'al Derech ha'Peshat', who attributes this to the physical change that takes place in a person when he reaches the next ten (20, 30, 40 [as described by the Mishnah at the end of the fifth Perek of Avos]).
See Ramban, who refutes this explanation, which he cited in the name of the Ibn Ezra.