Edward Marks, “Life Expectancy” Doesn’t Measure How Long You’re Expected to Live

To complete the picture we need to estimate what age-specific death rates someone born in 1940 will face in the coming years. As I’ve written before, the mortality rate for any age decays exponentially, the rate of progress is relatively stable for any age, and we’ve made a lot more progress for younger ages than older ages. I’ve plotted the average yearly progress for each age below.

Before: I Don’t See Any Evidence That We’ll Live Hundreds of Years

The reality is actually worse than this makes it sound, because while the average progress across all ages is 2%, these gains are not evenly distributed. Most of them go to people younger than yourself. For instance, progress for a child in its first year is about 5% per year, whereas progress for an elderly person in their 100th year is about 0.1% per year. This means that the life curve isn’t so much flattening out or even shifting out but rectangularizing. More and more of the population will live healthily to an old age, but they’re not going to start living hundreds of years. At least not in our lifetimes.

...To know where these curves will go in the future, we take a look at the rate of change in the past. To do this we calculate the percentage change between each year, averaged across all ages. While it does vary a bit over time, it’s actually remarkably stable, which means smooth exponential decay. Each year the average chance of dying across all ages is about 2% lower than it was the year before. Just as importantly, while mortality rates have fallen dramatically, the percentage rate at which they are falling hasn’t changed much in the last 75 years. If anything, we’re making less progress each year as time goes on. While 2% per year might sound small, it’s not. A 2% decay per year leads a value to fall in half in about 34 years, and then to fall in half in the next 34 years after that, and so forth. But as I mentioned before this progress is not the same for all ages, with the chance of a 1-year-old dying falling in half roughly every 17 years, and the chance of a 70-year-old phone falling in half roughly every 53 years. Most of the gains we’ve made are in the first twenty years of life, with progress for older ages appearing to slow almost to a halt.