She pointed to human average life expectancy of the mid-30s back in the mid-1800s. By the early 1900s, we were expected to live until 47. Now, it's 79.

It's not just living longer that conspired to fundamentally transform us as a society, but a sudden drop in fresh blood.

"The combination of longer life expectancy and lower fertility rates combined to produce an aging population," Carstensen said, giving birth (so to speak) to a society with a sustained older population.

Scientific progress, naturally, has played a pivotal role is extending human life expectancy, but it goes far beyond medicine.

Advances such as pasteurized milk, electricity — which led to refrigeration and the systemized collection and disposal of waste, Carstensen pointed out, did as much as penicillin to keep our ancestors from killing themselves.

But as fertility rates fell, Carstensen said, we as a society also began to actively invest in children in the form of education and public schools, until today, when education plays a greater role in one's life expectancy than age.

But that was just the beginning. Along with an aging population, we've also grown into a more diverse one, both in terms of a shifting population composition as well as divergent age structures by race.

So much so, that the white population of this country is actually expected to shrink over the next 35 years, and while the black and Asians populations are expected to hold steady, Hispanics will grow and get younger.

"The question then won't be whether young people will support their elders," Carstensen said. "But will young Hispanics be willing to support old whites?"

But we're a society shifting in other ways. The United States also has more people living alone or married without children than married people with children. In fact, less than 20 percent of the country's household mix is what would be considered traditional households.

What are older people 

Yet another shift is in the older population itself, which is more educated and healthier than its predecessors.

But it's also a demographic that's more emotionally stable — with lower levels of anxiety and anger — than its younger counterparts. And while actual learning speed stalls with age, cognitive function actually improves with work. Which is good, since in 2020, 35 percent of men and 28 percent of women are projected to still be working.

One of the things we should embrace as a society is that work will benefit them as much as employers, since Carstensen pointed to research suggest that, "not only will older workers be productive, but they'll help make younger workers more productive."

But just as she leans toward the optimistic, Carstensen quickly snaps back to the pragmatic.

"Workers are unable to finance a 20-year retirement on 40 years of work," she explained. "Planning for 20 years just isn't enough anymore."

So, sure, we've got to embrace different retirement models, embracing auto enrollment and auto escalation, but "we've got do better than telling people they need to save for retirement for health care," Carstensen said.