SAN ANTONIO, Texas. Looking over his shoulder, I can't help thinking this must be Johnny Mnemonic: The Early Days. The man alternately smiling and grimacing, speaking quietly to the big luminous Mac screen, is dipping his hands into the Great Data Pool--- ocean is more like it--- that is the U.S. Census report. He cuts and pastes, adds and sorts with a happy, practiced confidence.

His name is Ben Bradshaw. He is a professor at the University of Texas School of Public Health. His specialty is demography, the science of population study. I've come to receive an idiosyncratic guided tour of who we are and how we change.

Why am I doing this?

We are an aging nation. I know it's going to change our future, including the financing of Social Security and Medicare, but I want to see how our aging is distributed geographically. More important, demography is one of the few aspects of the future that we can actually know. If you want to know how many 55 year olds there will be in 2050, the number isn't science fiction. It's history. Just check the number of babies born in 1995. Make some good guesses about deaths. Adds some estimates for immigration. The result is the number of 55 year olds in 2050.

Professor Bradshaw's forte' is Texas counties. He likes studying them, he explains, because changes at the county level can be really dramatic. Much, he observes, gets smoothed out at the state level. Even more at the national level. To illustrate, he reminds me of an earlier visit where he showed me the age distribution, by race, in a Texas border county. The Anglo population was older, with few children. Not many young people. The Hispanic population was younger, with many children. Lots of young people. The age vs. population curves literally went in opposite directions.

"Is there a simple indicator of aging?" I asked.

"Oh, yes. It's the percentage of people 65 and over to those under 18. It's a lot more useful than the median age. The median just splits the population in half. You can have a lot going on and not know it from the median," he answered. He printed out a file showing the states, rank ordered by their "Aging Index."

"An index of 100 means the groups are about equal in size," he said. "If the index is 200 that means the odds are two to one that when you go outside you'll see an old person rather than a kid."

Florida, West Virginia, Pennsylvania, Rhode Island, and Maine are the oldest states, with aging indices running from 77.0 (FL) to 60.9(ME). Alaska, Utah, Texas, Georgia, and Colorado are the youngest states, with aging indices ranging from only 18.7 (AK) to 37.8 (CO). In most of America, you're still more likely to see a kid than an oldster. That will change. While some states will remain younger than others, the entire country is supposed to be as old as Florida by 2030. Later in the century, oldsters will outnumber kids. Indeed, if you look at young kids--- those under age 5--- America is already well into its Foggy Period. In 1900, kids under 5 were 12 percent of the population. They outnumbered oldsters three to one. By 2000, kids under 5 were only 6.8 percent of the population. There were nearly twice as many oldsters.

Those, however, are national figures. There are areas in virtually every state in America where you will see no children, but many retirees."

"How rapidly can a smaller area age?" I asked.

"It can take place very quickly. In Williamson County (Texas), for instance, a large retirement community opened. It showed up in the Census data," Professor Bradshaw said.

In fact, Williamson County is where Del Webb located Sun City-Georgetown. It also happens to be near the corporate headquarters for Dell Computer, which may explain why few think of the area as a "retirement destination."

The migration of seniors to Williamson County, however, was so great between 1995 and 2000 that it was one of only five counties in the entire country cited in a Census Bureau paper on elderly migration. The others were Maricopa County Arizona (Phoenix), Palm Beach County Florida, James City County Virginia and Nye County Nevada (Most of Nye county is public land now used as a dumping ground for nuclear waste but it abuts the exurbs of Las Vegas in Clark county.).

The same study showed that Florida ranked only third as a Geezer Magnet. Arizona had a greater net migration rate. But the top Geezer Magnet in America was Nevada--- its net oldster immigration rate was twice Florida's.

Professor Bradshaw observed that while the counties along "the spine" of Texas--- Interstate 35--- tended to be growing and young, many of the adjacent agrarian counties were aging and shrinking.

"How do you measure that?" I asked.

"With the Vital Index. It's the ratio of births to deaths. When the index is less than 100 there are more deaths than births," he said. Combine a high death rate, a low birth rate, and net out-migration, he observed, and an area can shrink and age rapidly. Dipping into the data, Professor Bradshaw showed me that 70 of the 253 counties in Texas had a vital index under 100. Many were older than Florida. In those areas, the future depends entirely on whether people are coming or going. Is there some neat bottom line to all this?

Sorry, no. All we know for certain is that while most people are looking at the growth and change in major cities around the country, pitting one city against another for jobs, the best way to anticipate how aging will impact the country may be to look elsewhere---to study the exurban agricultural areas.

Some are, today, far older than Florida.