What is the meaning of life expectancy?

Life expectancy is the expected lifespan if we are born this year. Sometimes I wonder, how do people know the expected lifespan into the future. They don’t! They use this year’s mortality rate as an approximation of future life expectancy.

For example, this year six person died. Three died at 80. Two died at 75. One died at 70. The average death age is (80*3+75*2+70*1)/6= 76.7. This number is used as the life expectancy.

Is the measured life expectancy closed to expected lifespan of people born this year? If the future environment is similar to current environment, probably life expectancy provides a good approximation to how long people might live into the future. However, if the future environment turns out to be very different, there is really no guarantee how long one can live matches life expectancy.

For example, in a long stretch of peace, such as today, life expectancy can become very high. However, the population and tension accumulated over long term peace will eventually lead to an outbreak of a great war, which will drastically reduce the actual lifespan of many people.