Many age-associated conditions, such as the decrease in regenerative capacity of tissues, appear to be determined by a decline in the function of specific somatic stem cells. Although it is obvious that the genotype determines the average lifespan of different species, the variation in lifespan of individuals within a species seems to be more affected by the accumulation over time of molecular errors that compromise adult stem cell function. These molecular alterations can occur at both the genetic and epigenetic levels and depend on hereditary, environmental, and stochastic factors. This complex multifactorial mixture determines characteristics, such as longevity and a healthy life, that are central concerns of human existence.