Projections of duration of life for humans based on mathematical models have led some researchers to claim that there is no lower limit to death rates or upper limit to life expectancy, and that a life expectancy of 100 will be achieved in the 21st century. To assess the biological plausibility of these claims, we examined temporal aspects of biological phenomena in three mammalian species. Our examination revealed that: (1) physiological declines associated with reproduction consistently occur at ages that are less than one-third of the median age at death, (2) physiological parameters associated with aging in humans lose eighty percent of their functional capacity by age 80, and (3) young versus old individuals can be distinguished by the pathologies detected at death. The biological evidence suggests that organisms operate under warranty periods that limit the duration of life of individuals and the life expectancy of populations. We use these findings to discuss the issue of limits to the duration of life and the validity of mathematical models used to forecast human longevity.