I am sceptical about statisticians' claims that, for example, average life expectancy for a baby born this year is x years, as opposed to y years for someone born, say, thirty years ago. Do these people have crystal balls? I suspect they just extrapolate recent trends indefinitely into the future.
To calculate any such values, certain assumptions have to be made. They have to estimate the probability of, for example:
1. A pandemic of a disease such as bird flu.
2. Nuclear holocaust or the use of other weapons of mass destruction.
In contrast, longevity of people who died last year, or in any other year for which statistiics are readily available, can be calculated with great accuracy, and analysed in great detail, as shown above.