We generally think in terms of life expectancy, but that's only really useful on a population level. On an individual level, it's much more useful to think about the probability of dying - you aren't running down a clock, you're constantly rolling cosmic dice to see whether you get hit by a semi truck or develop pancreatic cancer.

Before the age of 40, you've got less than a 1% chance of dying in any given year. By 60 that probability increases to about 5% and by 80 to about 25%. Some young people will just have rotten luck and roll 1 on a d100, while some people will repeatedly roll a d4 and manage to dodge the 1. Obviously those probabilities are highly modifiable by many factors, but some people will get unreasonably lucky or miserably unlucky regardless of the underlying probabilities.