Life expectancy

Life expectancy is the average number of additional years a person could expect to live, calculated from the mortality rates observed for each age group in a given year. It is most commonly reported "at birth," but can be calculated starting from any age, such as life expectancy at 65, which is used heavily in pension and retirement planning.

The standard figure is period life expectancy: a hypothetical measure built from a life table that applies the mortality rates of a single calendar year across an entire lifetime, as if those rates never changed. This differs from cohort life expectancy, which tracks the actual mortality experience of people born in a given year as they age through real, changing conditions over decades; cohort figures are only fully knowable long after the cohort has largely died, so period life expectancy is the number almost always reported and compared across countries and over time.

Life expectancy is a core input to the Human Development Index and is tracked alongside fertility rate and migration flows to understand a population's population pyramid and its dependency-ratio trajectory. The common misreading is treating period life expectancy as a prediction of how long someone born today will actually live: because it holds current mortality conditions fixed, it does not account for future changes in medicine, public health, or living conditions that will very likely alter real outcomes for anyone born today.