Average age at death in infancy and infant mortality level: Reconsidering the Coale-Demeny formulas at current levels of low mortality

<b>Background</b>: The long-term historical decline in infant mortality has been accompanied by increasing concentration of infant deaths at the earliest stages of infancy. In the mid-1960s Coale and Demeny developed formulas describing the dependency of the average age of death in infancy on the level of infant mortality, based on data obtained up to that time. <b>Objective</b>: In the more developed countries a steady rise in average age of infant death began in the mid-1960s. This paper documents this phenomenon and offers alternative formulas for calculation of the average age of death, taking into account the new mortality trends. <b>Methods</b>: Standard statistical methodologies and a specially developed method are applied to the linked individual birth and infant death datasets available from the US National Center for Health Statistics and the initial (raw) numbers of deaths from the Human Mortality Database. <b>Results</b>: It is demonstrated that the trend of decline in the average age of infant death becomes interrupted when the infant mortality rate attains a level around 10 per 1000, and modifications of the Coale-Demeny formulas for practical application to contemporary low levels of mortality are offered. <b>Conclusions</b>: The average age of death in infancy is an important characteristic of infant mortality, although it does not influence the magnitude of life expectancy. That the increase in average age of death in infancy is connected with medical advances is proposed as a possible explanation