On a bright February morning under a blank desert sky, three experts in world population get into a van in Tempe, Arizona, and drive back to the future. From the campus of Arizona State University on the edge of Phoenix, the three head northwest along Grand Avenue, following old U.S. Route 60 out of the city. On either side, what used to be cotton fields and cattle feedlots, and before that catclaw bushes and cactus scrub, has turned into suburban sprawl. The Phoenix metropolitan area, a.k.a. the Valley of the Sun, has grown more quickly than any other urban area in the United States, following an influx over the past decade of Hispanic migrants and white retirees. Due largely to the latter, the Northwest Valley of Phoenix is one of the fastest-aging population centers of the country.

The day starts cool, even cold. Frost disrupts the tee times on the bright green golf courses dotting the Northwest Valley. Arizona’s median age is 34, but at the point where Grand Avenue crosses the dry bed of the New River, palm trees sprout from the sidewalks and the median age jumps to 75. Silver-haired drivers on souped-up golf carts nose into the traffic, one maneuvering fearlessly in front of the university van. Screened by a low white wall, rows of nearly identical single-level houses nestle on tidy, concentric streets. A big hospital overlooks the development like a lifeguard scanning a beach. Welcome to Sun City, Arizona, population 38,000, the once and future retirement mecca, where the whole world seems to be headed.

At Del Webb Boulevard—Del E. Webb was the visionary developer who built Sun City—the van turns and parks in front of the community’s historical museum. The three academics get out. They are Michael Birt, 58, a gerontologist and director of the university’s Center for Sustainable Health; Jennifer Glick, 42, a sociologist and demographer at the ASU Center for Population Dynamics; and Haruna Fukui, 32, a Japanese graduate student working on her Ph.D. in sociology with Glick as her adviser.

At DISCOVER’s invitation, the trio had formed an impromptu panel. They were asked to discuss global population trends, including growth, fertility, and the impacts of immigration. But especially they planned to address the overarching trend of aging, which some researchers are calling “the gray tsunami” because it threatens to inundate the world’s health-care systems and sweep away today’s social, political, and economic norms. To make the discussion more pointed, it would take place during a field trip to Sun City, the prototypical American retirement community, now entering old age itself. None of the three has been here before, and they are curious to see it.

Population growth, not aging, has drawn the lion’s share of public attention, so the panel speaks to that topic first. There was consternation in the media when the Population Division of the United Nations announced that Earth had gained its 7 billionth person in 2011. By 2050 there could be 3 billion more of us, according to the agency’s most pessimistic projection. But Glick, the demographer, says, “Let’s not make a big deal about that number. The focus should be on the rate of growth and on the eventual turnabout.” Although billions of people are still in the pipeline, global population growth is slowing so rapidly that a decline in the population later this century seems unavoidable.

Demographers habitually take the long view, because subtle changes in population trends may take 50 years or more to register statistically. In 1960, the year Del Webb sold the first homes in Sun City, demographers believed that Earth’s population was out of control. “We thought it would explode,” Glick says. The sense of gloom was captured in 1968 by Paul Ehrlich’s best-selling book, The Population Bomb, which predicted widespread famine and mass mortality. Instead, an annual population gain of 2 percent has been cut in half and continues to head down. “That’s because fertility has declined, which writers of the time didn’t anticipate,” Glick adds.

The slowdown in growth has been offset somewhat by rising longevity. “People are living longer, 20 and 30 years longer, across the globe,” observes Birt, the gerontologist. Greater longevity causes a society to age unless births surge in compensation. In fact, the U.N. called attention to the aging phenomenon in developed nations as early as 1956.

Aging Boom / Fertility Bust

Del Webb was no demographer, but in the late 1950s he saw an opportunity in America’s budding crop of elderly. Promoting the then-novel idea of “active retirement,” Webb was a very active 60-year-old himself. Tall and lean, a vigorous golfer and baseball fan, he was a millionaire contractor with a common touch. The people who flocked to see his Sun City demonstration homes—100,000 showed up over New Year’s weekend in 1960—had had their fill of hard times. These were people who had lived through an economic depression and a world war. The advertisements for Sun City depicted a golden way of life in a place where they could retire and relax, where they would not be frail or sick.

Some of those ads now hang in the Sun City Historical Museum, which occupies one of the first homes to be built here, next to the first golf course. Two vintage golf carts, labeled Him and Her, stand side by side in the carport. Inside, the modest fixtures and furniture of a typical 1960s retired couple are on display. The original cinder-block structure consisted of five rooms totaling just 858 square feet; an addition was put on the back later. The small eat-in kitchen features a boxy electric range and fridge. The sink in the pink-tiled bathroom is very low and the toilet is minuscule, hardly suitable for today’s amplified Americans. The three academics smile as they look into the bathroom. “There are no handrails, nothing to grab onto,” Glick says.

Sun City’s radical idea—to restrict home ownership to people 55 and older—effectively excluded families and children from the development. But recently the policy was updated. Now only one owner has to be over 55, this to accommodate residents with younger spouses. Getting back in the van and touring the quiet, curving streets, with their neat plantings and pink-tinted gravel, the ASU group sees no pregnant women or kids, no young people whatsoever. Sun City has a fertility rate of zero.

The fertility rate is the number of children an average female will produce in her lifetime. The panelists note that the rate is currently plunging in almost all countries around the world. True, it has not occurred in sub-Saharan Africa, not yet. But for those who specialize in the long view, fertility collapse and accelerated aging have supplanted overpopulation as the most salient demographic trend.