The ratio of elderly to working-age adults is more than a number. It is also the fuel for political debate over federal entitlement programs for the elderly and a key to understanding consumer demand in any market. Nationally, the ratio will begin to grow rapidly when the oldest baby boomers reach age 65 in 2010, and this has dire implications for Social Security. But the boomers' retirement won't turn every city into Sun City. Aging will be dramatic in places with few children, but some of today's retirement zones will get younger.

Aspen, CO (Pitkin County, 1996 population 14,160) has only 6 elderly residents per 100 workers, compared with a national average of 27 per 100. This ultra-affluent resort also has just 22 children per 100 workers, compared with a national average of 47 per 100. If Aspen's boomers stay in town, the county's lack of children could cause its old-age dependency ratio to increase more than fivefold by 2020. Nationally, the ratio is projected to increase about 42 percent, according to Woods & Poole Economics.

Military bases also have relatively few elderly residents. The lowest elderly-to-worker ratio in the country is in Fort Benning, GA (Chattahoochee County, pop. 15,600), with just 3 elderly per 100 workers. While that ratio could reach 12 per 100 in 2020, Fort Benning will still seem young compared with a projected national ratio of 37 per 100.

In many Florida retirement counties, today's elderly-to-worker ratio exceeds the projected national ratio for 2020. The oldest county is Highlands, FL (pop. 74,850), with 73 retirees per 100 workers in 1996. And if northern snowbirds keep flocking to the wide-open palmetto prairies of Flagler County (pop. 40,480), the ratio could increase from 51 per 100 now to 117 per 100 in 2020. But most of Florida's major metros won't see such dramatic change. And in a few places, such as Fort Lauderdale (Broward County, pop. 1,441,780), workers may gain ground as young Hispanics and other migrants overwhelm a fixed population of retirees.

The population in more than 150 counties could get younger as America ages. This isn't always good economic news: the Great Plains has been losing workers for decades, and counties like Osborne, KS (pop. 4,600) are dominated by elderly natives who are "aging in place." As this generation passes away, places like Osborne could become younger, smaller, and poorer. There are rural counties with lots of children, such as Mormon-dominated Beaver County, UT (pop. 5,210). And suburban behemoths like Riverside County, CA (pop. 1,406,440) will stay young if they remain attractive to working families.

The ratio of elderly to working-age adults is a crude measure of economic dependency, because some people work past age 65 while others aged 18 to 64 are not in the labor force. An increase in this ratio won't necessarily bring economic ruin, either. Social Security could be saved by a combination of political reform, boomers delaying their retirement, and a rapid increase in the economic output of workers. Children will also consume less of society's resources, because the ratio of children to working-age adults is projected to decline 11 percent between 1996 and 2020. What is certain for many markets is a massive shift in focus toward the concerns of aging. To see the future of Colorado, look at Florida.

As boomers age, the diversity of this large group grows increasingly clear. "People talk about the 78 million boomers as though somehow they came out of the chute at the same time," says David B. Wolfe, an author and consultant based in Reston, Virginia. The boomer term is "mostly meaningless," he adds. Deciphering the factors that determine boomer behavior can help businesses predict what this group will want and need in the future. This year, the nation's 78 million boomers are aged 31 to 49. As a result, they are much more likely than either younger or older adults to have dependent children at home. Despite delayed marriage and high divorce rates, boomers these days are most likely to be part of a married-couple family. Even among the youngest boomers, nearly two-thirds are currently married. Sixty-two percent of adults aged 30 to 44 have children under age 18 at home, compared with 37 percent of those aged 18 to 29 and 28 percent of those aged 45 to