This article develops and applies two expressions for the rate of change of a population's mean age. In one, aging is shown to be negatively related to contemporary birth rates and death rates. In a general sense, aging occurs when vital rates are too low, as illustrated through applications to the United States, the Netherlands, and Japan. The other expression relates the rate of aging to a population's demographic history, in particular to changes in mortality, migration, and the annual number of births. Applications to the United States and Sweden show that the dominant factor in current aging in these countries is a history of declining mortality. Migration also contributes significantly but in opposite directions in the two countries. The two approaches are integrated after recognizing that the rate of change in the mean age is equal to the covariance between age and age-specific growth rates. A decomposition of this covariance shows that the two seemingly unrelated expressions contain exactly the same information about the age pattern of growth rates.