Despite advancements in aging research, we lack basic understanding of the biological processes that shape senescence. Here, we show that for a simple isogenic bacterial system, Escherichia coli, in a controlled environment, senescence results from two stochastic processes: a primary random deterioration process within the cell underlying classical senescence patterns; and a secondary process of stochastic asymmetric transmission of a factor influencing mortality at cell fission. This second process is required to explain the near non-senescence of old mothers offspring and the lack of a mother offspring correlation in age at death. We observed that life span is predominantly determined by underlying stochastic stage dynamics. Our findings do not support evolutionary postulates that base their reasoning on the age-specific action of alleles, but could support stage-specific actions of alleles.