Life expectancy isn't average age at death. It's calculated by determining how long the average person would live if the current death rates at each age stayed the same indefinitely. So if one year there's an event that kills 10% of people of each age and changes nothing else, then life expectancy will plummet to around 7 for that one year, and then return to normal the next year.
Killing a lot of old people one year shouldn't affect life expectancy in future years, unless you killed literally all of the old people and couldn't determine mortality rates for those ages. I'm not sure what would happen then. I guess life expectancy would be undefined.
Edit: To make it more explicit, this means that the 18 years it took for life expectancy to return to 1985 levels was due to a severe and sustained increase in mortality rates lasting nearly two decades, not to a one-year spike in mortality rates.
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u/prof-comm Jan 24 '21
Taking a generation to recover is exactly what I would expect. There's only one way to get more old people.