Gertrude Baines, age 114, is currently the oldest living human being. That is quite amazing, and yet, it has always been a custom of Jews to wish each other “ad meah v’esrim” - until 120.
One hundred and twenty years is considered the ideal human life-span, as stated in Genesis (6:3): “G-d said: ‘My spirit shall not abide in a human forever, since a person is also flesh; therefore a person’s days will be 120 years.’”
Adam, the first man, lived 930 years. And Methusalah, seven generations later, is renowned for his extraordinary longevity: 969 years. But by the era of the patriarchs, the normal human life-span had apparently dropped closer to a biblical life-span of 120 years. (Abraham lived 175 years, Sarah lived 127 years, Joseph lived 110 years, etc).
Only one person in the Torah lived exactly 120 years--to the day. According to the Midrash, Moses was born and died on the 7th day of Adar (which is today). Many understand the significance of his death at precisely 120 years to be a statement attesting to the fact that he had, without question, completed his life’s work.
One might ask how this could be true, since Moses did not merit to enter the “promised land.” While entering the land of Canaan was his life’s dream, it was not his life’s work. For the benefit of the Children of Israel and their future generations, it was necessary that the leadership be transferred. Moses, however, had led the Jews out of slavery, received the Torah for them at Sinai and judged and defended them throughout their journey in the wilderness. Through all this, Moses attained a height of communication with the Divine that had never been reached before, and never since. That was his life’s true work.