Extensive demographic studies relate aging to the increase in mortality, terminated by the species-specific lifespan limit. Meanwhile, recent experiments demonstrate that medfly mortality decreases at older ages, and challenge a limited lifespan paradigm. This paper proves that there exists a genetically programmed probability to die at any given age, and presents its phenomenological theory. The implications of the universal mortality law crucially depend on the cohort heterogeneity. For relatively high heterogeneity the law predicts unitarily vanishing old age mortality; this is verified with medfly data. For relatively low heterogeneity it predicts a precipitous drop in mortality fluctuations in old age. This is verified with demographic data. If comprehensive studies verify a species-specific characteristic age, then that age may be genetically manipulated. If the studies verify a unitary law of mortality, the results may be generalized to all species. A phenomenological model of mortality is presented.