On this crowded, hot, trampled planet, one of the most vexing trends is something countless numbers of us see when we look in the mirror: We’re going gray.

The United Nations has declared that the human population will hit 7 billion Monday, and an expanding percentage of those people are in the market for reading glasses.

The aging of the human race has been faster than anyone could have imagined a few decades ago. Fertility rates have plunged globally; simultaneously, life spans have increased. The result is a re-contoured age graph: The pyramid, once with a tiny number of old folks at the peak and a broad foundation of children, is inverting. In wealthy countries, the graph already has a pronounced middle-age spread.

This is, in many respects, very good news. Longer life is a blessing of modern medicine and improvements in nutrition. Lower fertility rates have corresponded to more educational opportunities for women and greater prosperity for societies in general.

But the unexpectedly abrupt demographic transition has created economic upheaval. For the countries that hit the fertility brakes the hardest, the graying of society has become a full-blown crisis. They’re suddenly desperate for babies. They need more workers to provide goods and services to huge numbers of pensioners.

The fertility rate in Germany, Italy, Spain, Greece and many other nations is less than 1.5 children per woman, dramatically lower than the “replacement” rate of 2.1 children (the extra 0.1 accounts for children who do not survive to adulthood). Japan (fertility rate 1.4) is already the oldest country in the history of the world; South Korea (1.2) is not far behind. China (1.5) is racing to get rich before it becomes old.

In far better shape demographically is the United States, with a fertility rate just slightly below replacement level. Immigration boosts the workforce. But the
baby-boom generation is storming the higher age brackets; the number of Americans 60 to 64 jumped from 11 million to 17 million in the most recent census. When Social Security was established in 1935, life expectancy in the United States was just under 62 years at birth. Today, it is 78 and rising.

The precipitous drop in fertility in many nations caught demographers by surprise, said Linda Waite, director of the Center on Aging at the University of Chicago. No one realized until relatively recently that fertility rates would keep dropping even when women began having fewer than two children, she said.

“It’s sort of a head slap,” Waite said. “It wasn’t even talked about. It was more an unspoken assumption that fertility would fall to replacement and then stabilize.”

“There are many countries, more all the time, that are going to be looking at a population implosion, rather than a population explosion,” said Matthew Connelly, a Columbia University professor of history and the author of “Fatal Misconception: The Struggle to Control World Population.”

The aging of the world will change cultures in myriad ways. People may have to extend their working lives far beyond the traditional retirement age. Countries may start competing for immigrants. Across the planet, vast numbers of people already are migrating from high-fertility countries to those that need workers.

The planet as a whole doesn’t have a baby shortage. Every minute of every day, according to the Population Reference Bureau, the number of births exceeds the number of deaths by 158. But the growth isn’t spread evenly. Of the net increase, 154 are in the developing world.

Ethi­o­pia and Germany have roughly the same population today, but Ethiopia’s is expected to more than double in the next four decades while Germany’s is projected to shrink by 10 million people.

The demographic transition is a significant factor in the financial crisis in Europe and the ongoing debt debate in the United States. In both places, the number of workers will steadily and dramatically decrease in relation to retirees. In the United States, the ratio of working-age people to retirement-age people will go from about 5-to-1 to 3-to-1 in the next two decades, according to the Census Bureau.

And America is a juvenile country compared with Japan, where, by mid-century, the 65-plus cohort will reach 40 percent of the population. If the trend holds, there will be just one working-age person per Japanese retiree.

“It’s a big, big social change. Lots of things are going to be disrupted,” said Ted C. Fishman, author of “Shock of Gray,” a 2010 book whose subtitle frames the issue comprehensively: “The Aging of the World’s Population and How It Pits Young Against Old, Child Against Parent, Worker Against Boss, Company Against Rival, and Nation Against Nation.”

Fishman points out that this isn’t all bad, although it is a challenge:

“Longer life is what human beings have wanted ever since we started talking to spirits and mixing herbs in bowls. And we worked at the top of our intelligence to get to this point of our life. It took almost the sum total of human history to get it. And now we have to work at the top of our intelligence to solve the social challenges that come with longer life and aging societies.”

The global transition to lower fertility has many factors, including a basic change in attitudes toward children. In societies with high infant mortality or without social programs to support the elderly, parents desired many children as a kind of insurance. And in some countries, having lots of children was considered a patriotic duty. For example, at one point under Mao Zedong, China made heroines of women who had large families. In the 1970s, China abruptly reversed course and instituted a coercive one-child policy that sent the nation’s birthrate into free fall.

It’s unclear how big the human population will get. The planet added a billion people, net, in the past dozen years (and is now more than twice as crowded as it was when Barack Obama was born). The most likely scenario, the United Nations said, will put the population at 10 billion at the end of this century and growing only modestly. But a relatively small uptick in the predicted fertility rate could result in a world with 16 billion people; a down-tick could mean a drop to 6 billion.

Another uncertainty is the extent to which environmental problems and resource scarcity will bend the population curve. People are contributing to climate change, acidifying the oceans, clearing the rain forests and pushing species to extinction — among other behaviors that do not appear to be sustainable over time.

The environmental footprint of people in wealthy, high-consuming, low-fertility countries is greater than that of people in poorer, high-fertility nations. That raises a philosophical question: Is a baby primarily a future consumer of precious resources on an already overstressed planet, or primarily a future producer of goods and services that sustain an economy — one with a growing cohort of people past their working years?

The answer in many aging countries is emphatic: Babies wanted. Pro-natalist policies — government-funded child care, tax breaks, cash payments for additional births — have proliferated in many European countries. National leaders in many countries recognize that cultural and economic factors are making it too costly to bear children.

In Russia, with a birthrate of 1.5, the government has offered women the equivalent of $9,000 in cash to have a second baby. On Oct. 20, Prime Minister Vladimir Putin traveled to St. Petersburg to observe the ceremonial arrival from Greece of the Belt of the Mother of God, according to Agence France-Presse. It is an Orthodox relic believed to have belonged to the Virgin Mary and to have powers to boost fertility.

Hundreds of Russians, mostly women, stood in line to see it.

This is the last in a series of articles exploring what reaching the 7 billion population mark means for the planet and its people. To read the other parts in the series, go to wapo.st/7billionpeople.