This Dvar Torah comes from Rabbi Jay Kelman of Torah in Motion. Since we just completed reading the Torah and begin again, let us contemplate what makes for a good life, as Rabbi Kelman cites in this drash.
"And Moshe was one hundred and twenty years when he died" (Devarim 34:7). It is a beautiful, if somewhat unrealistic, custom to offer blessings to those celebrating a birthday that they should live to be 120. While this quantity of life is (currently) unrealistic, the blessing to live to 120 relates not only to quantity, but to the quality of life; "his eyesight did not diminish and his strength did not wane" (ibid).
The Midrash notes that three other giants of Jewish history also lived to be 120; Hillel the Elder, Rabbi Yochanan ben Zackai, and Rabbi Akiva. And like Moshe, the Midrash claims that they, too, "gave sustenance to the Jewish people for forty years", forty being the number representing transformation. These four are the transformative figures of Jewish history.
It takes forty days for fetus to develop, the flood lasted forty days, Moshe spent forty days receiving the Torah, and we wandered for forty years in the desert, fulfilling G-d's promise to Abraham that we would be strangers in a land for four hundred years.
Not surprisingly, the Midrash divides the lives of these four heroes into three periods of forty years, with the culmination being their service to the Jewish people for forty years. Moshe, the Midrash notes, spent forty years in Egypt, forty in Midian, and forty "sustaining the people". Rabbi Akiva became interested in Torah at the age of forty, and Hillel arrived in Israel from Bavel at forty.
The message of the Midrash is not to convey their age at death, but to link these four great heroes of Jewish history. Each led at a time of great historical crisis, and they literally "sustained (parnesh) the Jewish people for forty years" (Sifri 36:7). Without their efforts, there would have been no Jewish history. Not only did Moshe redeem the people as they came perilously close to total assimilation--something the Midrash claims did indeed happen in the case of at least 80% of the people--he spared them from the destruction due them for their sinning. Hillel established the "house" (Beit Hilllel) that set up the contours of Jewish law. He was extolled for his great humility like Moshe, and had an uncanny ability to relate to all. His student Rav Yochanan ben Zackai perhaps single-handedly saved the Jewish people by not attempting to save Jerusalem, affording the opportunity to rebuild Judaism from the ground up in Yavne. With the Temple lost, many groups of Jews disappeared; and if not for Rav Yochanan's understanding that the Temple is only a means to an end, we would not be here today.
While Rav Yochanan ben Zackai saved Judaism, it was Rabbi Akiva who developed it. It was to his Beit Midrash that Moshe was "transported" at Sinai--to witness Rabbi Akiva "expound on every thistle and thistle, mountains and mountains of Jewish law" (Menachot 29b). His willingness to sacrifice his life in order to worship G-d with all his soul is the (tragic) model that was emulated by many. It is he and his students who are the primary teachers of the Mishnah and halacha.
Anthropologists generally divide life into three stages; growth, maturity and decline. Yet that is true in the physical realm only. If one is immersed in "sustaining the Jewish people", something each of us can do in some form or another, the legacy we leave for our people will endure for all time.
Even Moshe may not have been exactly 120 years old at death. The Torah tells us he was eighty when he first spoke to Pharaoh, and if we add the forty years in the desert, that leaves no real time for the ten plagues and the Exodus. Biblical numerology is often meant not as mathematical or historical certitude, but to convey certain ideas rooted in the symbolic nature of numbers.