Average Life Expectancy:

Let's work with a population of 100 to make it easy.

Let's say a normal person lives to 70 if they don't get killed.

But let's say 10 percent die at age 5.

So, now we have 90 people living to 70, and 10 people living to 5.

6,300 (90 x 70)

+500 (10 x 5)

6,800 / 100 = 68 years average.

The average life expectancy dropped 2 years just from these 10 people dying at age 5.

If we recalculate those 10, but this time they die the first week after they're born:

We get an average life expectancy of 63 now.

The other people are living to a ripe old age, it's jut the people dying young who skew the age downwards. And back in they day, both children and mothers often died during birth or from complications afterwards.

But if you made it to adulthood, you'd probably live to be an old person.

This would likely hold true all the way back to Paleolithic times.