You never get over the moment you realize that you're definitely going to die. You're usually a small child when the insight hits, and you usually have a vague idea of what death is, but the first-person epiphany — the "Wait, that's going to happen to me?" experience — changes everything. Your sense of time and its fleeting passage can never go back to what it was before you discovered that you too are on the clock.

Suppose, however, we could do even better. Suppose we could suddenly add another five or 10 or 20 years to American life expectancy. That would be a good thing, right? Maybe — or maybe not. We'd all like the extra time, but could our economy handle it? Our environment? What about our food and water supply? What about the millions of extreme oldsters themselves, not to mention their families? For every George H.W. Bush, who goes parachuting at 85, or George Burns, who performed his stand-up act at 95, there are many more sickly, bedridden nonagenarians who draw no joy from their added time, piling up medical bills and misery but not much more. Would extra years of life for all mean extra years of decay for most?

For TIME, Rand conducted one such FEM simulation. "We imagined that a pill emerged in 2012 that could increase life expectancy for every 50-plus American by 10 years," says Dana Goldman, a former Rand director and the current director of the Schaeffer Center for Health Policy at USC. "Then we ran the numbers to see what would happen."

According to the FEM model, the very year the imaginary long-life pill appeared, the over-65 population would jump 7% more than it otherwise would have, reaching 44 million. In 2014, it would be up 13%, to 49 million. By 2030, after the last boomer arrived in the senior set, the aged population would have swelled by almost a third more than it would have without the pill, hitting 85 million. The growth rate would not be quite as dizzying after that, but by 2080, the final year of the simulation, there would be 151 million 65-plussers in the U.S., or more than 43 million extra people, all of them old. That difference is nearly equivalent to the entire population of Spain.

Not surprisingly, this gray boom wouldn't come cheap. If life expectancy increases 10 years in 2012 and later, by 2028, annual Medicare expenditures alone would more than double relative to current levels, reaching $1 trillion. They'd double again to $2 trillion by 2050 and top out at $3 trillion in 2080. Total expenditures — including Social Security and disability — would be $10 trillion by that final year. In the real world, of course, life expectancy doesn't increase in the sudden way it does in FEM models, but it can increase fast — as America's addition of almost 30 years in a single century shows. We may not have to deal with the sticker shock of the FEM experiment, but the financial burdens on societies will inevitably grow as people live longer.

What drives up the medical portion of the costs so fast is that the mere ability to hang around till you're nearly 90 doesn't mean you're hanging around healthily. A study by the nonprofit Commonwealth Fund found that up to 20% of people entering Medicare are already suffering from five or more chronic conditions, such as hypertension or high cholesterol. Most of those ills are treatable, but not always inexpensively, and every extra year of life is another ring of the cash register. What's more, routine ills have a nasty way of turning into not-so-routine ones. "If people are getting older but still developing conditions at the rates we get them now," Goldman says, "you'd have half the elderly population suffering from heart disease."

Even when people do age healthily, that doesn't necessarily mean they're saving money. A 2005 Rand study looked at four common risk factors for health — hypertension, obesity, diabetes and smoking — and tried to determine what the cost effect would be if the conditions were partly or entirely eliminated among the elderly. The results were mixed. Eradicating hypertension would save Medicare a total of $890 billion in the 2005-to-'30 period. Cutting obesity 50% would save $1.2 trillion. Completely controlling diabetes, however, would actually increase spending by $246 billion. Eliminating smoking would boost costs by $293 billion. Why? Because diabetes and smoking cut off more years of life than hypertension or obesity. When you restore those years, seniors stay around long enough to make other claims on Medicare — and those claims may exceed the money saved by eliminating the original problems.

But wouldn't a nation of people who lived longer also work longer and thereby offset some costs by continuing to pay into the national tax kitty? Not necessarily. As it turns out, most people would choose to — or be forced to — punch out at normal retirement age anyway. According to the newer Rand analysis, an extra 10 years of life — assuming no change to Social Security or Medicare — would boost the Federal Government's income-tax haul just 3% by the mid-2030s and a mere 6% by 2080. "You could tell yourself a story that people would be productive longer," says Mazur. "The question is what kind of work would be available."

A different and often voiced concern is that those seniors who did continue to work would clog up the labor market, hanging on to jobs and denying them to younger people — a particular danger in a static or contracting economy. But Mazur believes that worry is overstated, in part because old people and young people rarely seek the same positions — though with manufacturing jobs being replaced by service jobs, the difference in physical abilities between the two groups is less important. Still, a likelier consequence of more seniors in the labor force would simply be that careers would play out more slowly, with what is now a 30-year climb up the company ladder perhaps taking 40 years. Younger workers themselves might ease the pressure by spending the early parts of their careers doing lower-paying, service-oriented jobs — say, for Teach for America or the Peace Corps — if they are given some incentive to do so.

Even if a 65-plus population explosion didn't make a mess of the job market, it could certainly help make a mess of the planet. The U.S. trash and carbon footprints are already huge, with every one of us throwing away roughly 1,679 lb. (760 kg) of garbage per year and generating 20 tons of atmospheric carbon. Now imagine adding 43 million more of us.

Older Americans do go a little easier on greenhouse gases than others, simply because they drive less — about 60% as much as middle-agers. But transportation in all forms accounts for only 29% of greenhouse emissions. Another 17% comes from heating, cooling and powering homes, a contribution we all make.

Garbage is also a scourge that knows no age. The exact things you throw away may change over the course of your life — more electronic trash like old computers or cell phones when you're younger, more grocery and pharmaceutical waste when you're older — but that's not the relevant concern. "The question is not so much what the mix is," says Elgie Holstein, a land, water and wildlife expert for the Environmental Defense Fund (EDF). "It's what the total quantity is."

Water is a different matter. Americans consume an average of 160 gal. (600 L) of water per person per day — at a moment when the overall supply of fresh water is dwindling because of a growing population and increasing droughts. The Colorado River, for example, is already being pushed beyond its capacity, required to serve 27 million people in seven southwestern states. By 2021 it will no longer be up to the job. Similar if not always equally dramatic challenges confront nearly every region of the country. "There are some parts of the U.S. that can handle only so many people," says EDF water specialist Spreck Rosekrans. "But there are still things we could do to stretch the water supply and support a larger population."

There are solutions — at least theoretical ones — to nearly all of the problems increased longevity would bring. For water, the answer is a mix of conservation and incentives to upgrade irrigation and industrial water systems, as well as better regulation of water markets so that wet regions can more readily sell their surplus to dry ones. The trash problem can be addressed by reducing packaging and boosting recycling; greenhouse gases can be slashed by switching to renewable energy and pricing carbon to make new technologies more attractive. Even the medical expenses associated with long life can be reduced by a combination of methods such as better preventive care and doing away with the fee-for-service model in favor of bundled payments for entire procedures. Medicare pilot programs for several such initiatives are already included in the health care legislation now before Congress.

And yet no matter what policymakers do to accommodate a bigger, older population, the hardest and most important work will still have to be done by the people who know the seniors best — their families. According to the U.S. Census Bureau, from 2000 to '07 the number of aging parents who have moved in with their adult children — so-called boomerang seniors — jumped from 1.4 million to 2.1 million, or 50%. That's still less than 1% of the population, but the trend is unmistakable, and a punishing economy combined with greater longevity means it's not likely to change soon. The emotional and fiscal stress this places on heads of households who often haven't even finished rearing their own kids can be enormous.