I've long thought that life expectancy statistics were odd. Encyclopedias treat nation's life expectancies as numbers on par with square mileage. But in reality, doesn't this require a bunch of assumptions about future health developments?
At least according to Wikipedia, demographers "solve" this problem by completely ignoring it. So-called "life expectancy at birth" numbers are mechanically calculated based on current deaths:
Life expectancy is by definition an arithmetic mean. It can be calculated also by integrating the survival curve from ages 0 to infinite (the ultimate age, sometimes called 'omega'). For an extinct cohort (all people born in year 1850, for example), of course, it can simply be calculated by averaging the ages at death.
Note that no allowance has been made in this calculation for expected changes in life expectancy in the future. Usually when life expectancy figures are quoted, they have been calculated like this with no allowance for expected future changes. This means that quoted life expectancy figures are not generally appropriate for calculating how long any given individual of a particular age is expected to live, as they effectively assume that current death rates will be "frozen" and not change in the future. Instead, life expectancy figures can be thought of as a useful statistic to summarise the current health status of a population. (emphasis added)
Bottom line: Unless you think that health won't improve in the future, official statistics are selling your longevity short. Plan accordingly!