The second time was in the past 150 years, when the average lifespan doubled in most parts of the world.
The first time was in the Paleolithic, probably around 30,000 years ago. That's when old people were basically invented.
Throughout hominid history, it was exceedingly rare for individuals to live more than 30 years, Stuff.co.nz reported.
Paleoanthropologists can examine teeth to estimate how old a hominid was when it died, based on which teeth are erupted, how worn down they are, and the amount of a tissue called dentin.
Anthropologist Rachel Caspari of Central Michigan University used teeth to identify the ratio of old to young people in Australopithecenes from 3 million to 1.5 million years ago, early Homo species from 2 million to 500,000 years ago, and Neanderthals from 130,000 years ago.
Old people - old here means older than 30 - were a vanishingly small part of the population.
When she looked at modern humans from the Upper Paleolithic, about 30,000 years ago, though, she found the ratio reversed-there were twice as many adults who died after age 30 as those who died young.
The Upper Paleolithic is also when modern humans really started flourishing. That's one of the times the population boomed and humans created complex art, used symbols, and colonised even inhospitable environments.
Caspari says it wasn't a biological change that allowed people to start living reliably to their 30s and beyond. Instead, it was culture. Something about how people were living made it possible to survive into old age, maybe the way they found or stored food or built shelters, who knows.
That's all lost-pretty much all we have of them is teeth-but once humans found a way to keep old people around, everything changed.
We're now on the other side of the second great demographic change in human evolutionary history.
The main reason lifespan doubled in the past 150 years is that infant mortality plummeted.
Just as having old people around changed human culture profoundly 30,000 years ago, having infants and children survive has fundamentally changed modern society.