Fewer babies were born in 2017 than in any year in the past three decades, according to data released this week by the Centers for Disease Control. That finding was unusual, given the strength of the economy and given population booms often track with economic strength. It is also remarkable because the decline in birthrates happens at the same time the population of older Americans keeps growing larger.

At a rate probably without precedent in American history, the number of older Americans is catching up with younger Americans — and is projected to overtake it within three decades.

Thanks to lower life expectancies a century ago, there were relatively few Americans 65 and older. The population aged 18 and younger made up a substantial portion of the population.

Over time, that older population grew much larger.

That is more clear when looking at those populations as a percentage of the overall population. In 1900, those 18 and under made up more than 40 percent of the population. People 65 and older made up less than 5 percent. In 2016, that gap had narrowed substantially: Less than a quarter of the population was aged 18 and younger, and more than 15 percent was 65 or older.

In part, that is because of the aging of the baby boomers, who started turning 65 in 2011.

Another way to look at this is by considering how many people 18 and younger there are for every person 65 and older. In 1900, there were 10 people no older than 18 for everyone who was at least 65.

In 2016, there were 1.6 18-and-younger for everyone at retirement age.

Framing it as “retirement age” reinforces one of the problems with the low birthrate: As more Americans age out of the workforce, there will be an imbalance between those paying into social security programs and those relying on them.

By 2042, according to Census Bureau projections, the number of people aged 65 and older will pass the population aged 18 and younger.

By 2060, if current trends continue, there will be 1.1 people 65 and older for every person who is 18 and younger. It is hard to think there will not be dramatic changes to American politics and governance should that projection come to pass.