ALMOST EVERY HIGH SCHOOL has a senior prom, but Pennsylvania’s Altoona High may be the only one that throws a seniors’ prom. Twice a year, the students decorate the gym and invite the town’s senior citizens in for a Big Band-style dance. For the last one, more than 600 retirees, including several in wheelchairs, showed up to boogie the night away–at least until 9 p.m., when the crowd tends to nod off. ““We just want to show them how much we care about them,’’ says Alicia McDowell, 18, the student-council president. That might sound like an extraordinarily thoughtful sentiment for a teenager, but the fact is that in Altoona, seniors rule. People 65 and older make up 19 percent of the population, which is almost as large as the school-age set–and well above the 13 percent average nationwide. There’s an employment agency that places only older workers and a grocery store that offers senior discounts, just like almost every restaurant in town. The grocery, Jubilee, even hosts bingo at 9 a.m. every Wednesday to lure senior shoppers. ““They come in at 7 a.m. to get good seats,’’ says Linda Day, 27, who works in the bakery.

Altoona may sound like a retirement village that got lost on its way to Florida, but it’s closer to home than you probably realize. In a new book published this month called ““Gray Dawn,’’ Peter G. Peterson predicts that in less than 25 years, senior citizens will comprise more than 18 percent of the entire U.S. population–the same proportion as in Florida today. Put another way, that means that early in the next century, there will be more grandparents than grandchildren. Peterson, a former secretary of Commerce under Nixon, is primarily concerned with what the aging of America–a product of both longer life spans and falling birthrates–means for Social Security and Medicare. But the social ramifications will be at least as profound as the economic ones. Will all those seniors shift the balance of political power? How will Hollywood executives, funeral directors and the auto industry change their products to meet the demands of a markedly older public? Because women tend to outlive men, will an older America also be significantly more female? In short, what will America be like when we all become a Senior Nation?

Anyone who has visited West Palm Beach or Tucson knows part of the answer: lots of people driving very slowly in big cars on their way to early-bird dinners. But that’s only the most broad-brush observation. The political changes alone will be enough to bury all those stereotypes about the feeble elderly. Peterson estimates that by 2038, people 65 and older will make up 34 percent of the electorate–up from only 16 percent in 1966. You think Social Security is a sacred cow now? And the battle over entitlements may get uglier. The 65-plus population is about 85 percent white. The younger generations–the ones footing the seniors’ bills–are much more racially mixed. ““What you’ve got is an overwhelmingly white generation with enormous influence asking African-Americans, Hispanics and Asians to support them for decades,’’ says Ken Dychtwald, president of Age Wave, a consulting firm that focuses on the maturing marketplace. ““The tension becomes not only generational but racial.''

A big loser in the generational tug of war may well be education. Sci-fi writer William Gibson once said ““the future is already here, it’s just not evenly distributed yet,’’ and that’s certainly true when it comes to seniors and school politics. One retirement community near Phoenix already tried to secede from the Dysart Unified School District in 1997 in order to avoid paying school taxes. When James Button, a political-science professor at the University of Florida, studied the voting patterns of older people in six Florida counties, he found that they weren’t just hostile to schools. ““We had people saying, “I wish they’d stop building tennis and basketball courts and start building more shuffleboard and walking trails.’ You see how the young versus old is a significant factor.''

A grayer country is not necessarily a meaner one, however. Because young people commit a disproportionate number of crimes–teenagers kill 10 times as often as people over 50–a senior nation will likely be safer. It might be gentler, too. Women outlive men by an average of seven years, which raises the possibility of swaths of society dominated by grandmothers. ““Women have a different sensibility than men: more empathic, less of the macho gladiator. Does that mean there’ll be less war?’’ says Dychtwald. Grandma might not be thrilled with this arrangement. There is already a serious man shortage in the retirement areas of south Florida–10 women to every man, estimates Elaine Horvitz, president of the Jewish Matchmaking Co. Things are so bad at the Gold Coast Ballroom near Ft. Lauderdale that men get in free–and they get to literally go down the line, dancing with every willing lady in the place. Needless to say, most of the women’s dance cards are pretty empty. ““Older men want someone who can cook and make them coffee and take care of them,’’ says Ellie Luge, 69. ““I’d just like a full-time companion who isn’t looking for a nurse with a purse.''

Companionship isn’t the only commodity in short supply. The Census Bureau estimates that the number of people over 65 will increase from 35 million in 1995 to 53 million in 2020, and the business world is only beginning to notice that untapped market. Some of the industries likely to thrive are obvious: health care, golf-club manufacturers, funeral parlors. But people over 65 also buy half of all the luxury cars sold. They also tend to eat out more than the average American. In Altoona, a blue-collar town of 50,000, more than 20 new restaurants have opened in the last two years–though they’re much busier at the beginning of the month, when the seniors get their Social Security checks. Hollywood is in for a wake-up call, too. Movie studios and TV networks will do anything–even cast Shannen Doherty–to rope in the under-30 demographic. With the gray dawn, that’s bound to change, though that doesn’t mean a ““Golden Girls’’ on every channel. ““Just because I’m going to be 50 doesn’t mean I’m going to want to watch a movie where everybody is 50. But it does mean I’m not as silly as when I was 12,’’ says Dychtwald.

Demographers have long predicted that the country would undergo a major aging wave as the 75 million or so baby boomers barge their way into retirement–and then, aided by science, live longer than anyone has before. But Peterson argues that boomer longevity makes up only half the aging equation. Society is also getting older on average because we’re having fewer children. In 1960, the typical American couple had 3.7 kids, which front-loaded the country with young people. By 2000, the fertility rate is expected to hit 2.0 per couple. With- out a boost from immigration, the U.S. population would actually shrink at that level. The fertility rate is expected to edge up for a while, especially when the baby boomers’ children–sometimes called the baby boomerang–start to have kids in the next decade. But by then, the percentage of older people will already have shot up faster than an Internet stock offering. That’s already happening in Italy and Japan, where the fertility rates have fallen below 1.5. Italy will be Floridated–with more than 18 percent of its population over 65, just like in Florida today–in only four years.

This double whammy–more older people, fewer young ones–will likely create its own problems. There will be fewer siblings to take care of their aging parents, and the parents will be around longer than ever. That scenario might have an impact far beyond any sick room. ““Imagine where a sizable portion of the Army comes from a one-child family,’’ Peterson says. ““Would the parents even permit him or her to go off to war?’’ Buffalo provides another Gibson-like window into the future. Since 1960, its 65-plus population has increased 57 percent while the number of people under 30 has fallen 26 percent. Without young buyers for starter homes, the Buffalo real-estate market has frozen over. The median price of existing single-family homes flatlined over the last five years, while the rest of the country has seen one of the hottest real-estate markets in history. ““What are people going to do with those homes?’’ says Gary Keith, vice president of the Manufacturers and Traders Trust Co. in Buffalo. ““Who are they going to sell them to?’’ The concept of a house as a sure-fire investment may soon be over.

Gibson may have had a point, but looking at today to predict tomorrow isn’t foolproof. Even the most obvious assumptions could be wrong. ““You might say that the coffin industry is really going to boom, but in the future funerals will be on the Internet,’’ says Dychtwald. ““There will be a Ken Dychtwald.com site, with my favorite books and poems and some pictures of me that my great-great-great-grandchildren will be able to visit. If you can see how categories will be not only multiplied but transformed, you can see your way to a trillion dollars.''

It’s also possible that demographers have miscalculated entirely. An unexpectedly large influx of immigrants or an unforeseen jump in the birthrate could take some of the heat out of the aging explosion. Most experts believe the opposite is true, that we’ve underestimated the power of science to prolong lives. Carl Haub of the Population Reference Bureau says that by 2050, the average life expectancy for men and women could reach 89, up from 76 in 1995. That would mean the Census prediction of 79 million retirees in 2050 would be short–by 24 million. ““It could be a whole new mortality revolution,’’ Haub says. And that could turn the gray dawn into a gray morning, noon and night.

IT’S NOT JUST HERE The number of Americans over 65 will hit 18 percent in 24 years–and much earlier in other countries.

Italy 2003 Japan 2005 Germany 2006 United Kingdom 2016 France 2016 Canada 2021 United States 2023 SOURCE: “GRAY DAWN”

COUNTING THE SILVER HAIRS After the gray dawn, what will our days be like? Seniors will live longer–will they become a burden to their kids? There will be more women, but fewer of them will give birth. And in less than 40 years, seniors will be most powerful demographic in the nation.

Age of voters participating in United States congressional elections 1966 18-44 43.6% 45-65 40.4% 65 and over 16.0% 1994 18-44 43.4% 45-65 33.2% 65 and over 23.4% 2038 18-44 32.2% 45-65 33.7% 65 and over 34.1%