Moses’ census of the Jewish people, defined as a count of “all who are fit to serve in the army of Israel,” included only those who were “from the age of twenty and upwards.” What is the significance of this requirement?
The fifth chapter of Ethics of the Fathers includes an outline of the phases of a person’s education and life: “At five years of age, the study of Scripture; at ten, the study of Mishnah; at thirteen, the obligation to observe the mitzvot; at fifteen, the study of Talmud; at eighteen, marriage; at twenty begins the pursuit [of a livelihood]; at thirty, one attains strength; at forty, understanding; at fifty, one can give counsel . . .”
In other words, the first twenty years of a person’s life represent those periods and areas of his life in which he focuses almost exclusively on his individual growth: the acquisition of knowledge and wisdom, and his moral and spiritual development. “Twenty” represents the point at which he ventures out to the world and begins to concern himself with the material involvements of life.
Therein lies the deeper significance of G‑d’s instruction to Moses that only “from the age of twenty and upwards” shall a person be counted as one “fit to serve in the army of Israel.”
A period of intense self-development and spiritual self-enrichment is a necessary preparation to life, but it must not be seen as an end in itself. The purpose of the “pre-twenty” times and aspects of a person’s life is for the sake of the “pursuit” which must follow: that he or she go out into the world and apply his personal attainments to the development and sanctification of the material reality. One who does not graduate to the “post-twenty” phase of life cannot count himself as a member of the “army of Israel.”
— The Lubavitcher Rebbe