This comment is misleading and factually incorrect.

First of all, I did specify I was referring to life expectancy at birth, thus rendering your first remark useless from the get go; but to elaborate, throughout human history, from the Paleolithic (which was more than 2 millions years ago) through the bronze age, LEaB was on average 25 years - that covers 90% of our existence on Earth. Which is definitely most.

The Middle Ages are recent history on that scale. Even so, LEaB in the middle ages was 30.

Secondly, it is true that after you pass a certain threshold, LE extends, because it is presumed that you managed to avoid whatever perils that would kill you up to that point and you will do so more or less from then on, being somewhat in the clear. But that threshold is not the first months/years of one's life.

Furthermore, without nuance, this is misleading.

To elaborate:

I'll make up an example to facilitate the explanation: You are born, your Life Expectancy at Birth (LEaB for convenience) is 25 years. If you manage to get to 10, you get a few more years, and now your LE is 35. You get to 20, your LE is now 50!

BUT! Statistically, the chance of you getting to 10 is 0.8, to reach 20 is 0.35 etc, which means that there's a 20% chance you're dead by 20, and a 65% chance you're dead by 20, and this is how mortality factors into how LE are calculated!

It is not an average pushed down by children mortality. Children/infant mortality does weigh in, but not how the poster above interprets it.