A common procedure in paleodemography is to use mean sketetal age to estimate the expectation of life at birth in a population. This paper shows taht skeletal age and expectation of life at birth are not equivalent unless the population is stationary. this assumption is not justified for most real populations. we show that mean skeletal age is approximately equivalent to the reciprocal of the birth rate and is not correlated with the death rate. Thus, the practice of inferring changes in life span and death rates from changes in mean age at death is not reliable and most conlcusions of paleodemographic studies should be revised. On the other hand, skeletal age may provide high quality information about fertility in archaeological populations. Several published pelodemographic studies are reinterpreted in light of the model presented.