Commentaire sur Le Deutéronome 31:2

וילך משה וגו' ... אנכי היום AND MOSES WENT ... I AM [HUNDRED AND TWENTY YEARS OLD] THIS DAY — Today my days and my years become full: for on this day (the seventh of Adar; cf. Rashi on Deuteronomy 1:3) I was born and on this day I must die (Sotah 13b).

AND HE SAID UNTO THEM: ‘I AM A HUNDRED AND TWENTY YEARS OLD THIS DAY.’ This was to comfort them concerning his condition [i.e., about his approaching death], as if to say, “I am old and you have no more benefit from me. Moreover, G-d has commanded me that I should not go over there. Do not dread and do not be fearful, for the Eternal will go over with you; He will not remove His Presence from you on my account [i.e., because of my absence], and Joshua, he shall go over before³Further, Verse 3. you in my place.” Now, although Moses our teacher retained his vigor and health, as Scripture testifies, his eye was not dim, nor his natural force abated,⁴Ibid., 34:7. he told them this [“I am old and you have no more benefit of me”] in order to comfort them [over the transfer of leadership to Joshua].
Now, Rashi wrote: “I can no longer go out and go in, because the Eternal hath said unto me: Thou shalt not go over this Jordan.” But it is not correct.⁵The text reads: ‘Vashem’ (‘and’ the Eternal) said unto me. Now, Rashi explains that the connective vav, which generally means “and,” signifies “because.” It is this interpretation which Ramban considers incorrect because vav always means “and,” not “because.” Mizrachi, too, raised this objection to Rashi and left it unanswered. See, however, in Ma’aravi where a different version of Rashi’s text is cited, in which the connective vav has its usual meaning. And in the opinion of Rabbi Abraham ibn Ezra the expression I can no longer go out and come in means to war, because his powers weakened in his old age. This too, is not correct.⁶For Scripture testifies: his eye was not dim, nor his natural force abated (further, 34:7). And our Rabbis have said:⁷Sotah 13b. “This teaches us that the well-springs of wisdom were stopped for him.” This was a miraculous event in order that Moses should not be troubled [about the transfer of leadership to Joshua] and [G-d] bestowing honor upon Joshua in his [Moses’] presence.⁸See further, Verse 14.

בן מאה ועשרים שנה אנכי היום, Moses mentions his age to remind the people that there is no reason to be sad that someone of his age has to die. According to any life expectancy and law of nature, he was no longer fit to go on living by natural means.