How long can humans live?
The maximum human life span is currently about 125 years, scientists think. Most people fall far short of that because of poor diet, self-destructive habits, disease, or organ failure. But advances in medicine have already extended the average human life span in the U.S. and other modern nations from 46 in 1900 to 78 today, and science is now making steady progress toward solving the problem of aging itself. Through genetic manipulation, scientists are hoping to train the body's immune system to destroy cancer cells, and to tweak our DNA to prevent our healthy cells from dying. We may be approaching an era in which people can bring their aging bodies to a clinic for maintenance, like a car, and have new organs installed that were grown from stem cells or manufactured by 3D printers. "I'd say we have a 50-50 chance of bringing aging under what I'd call a decisive level of medical control within the next 25 years or so," says gerontologist Aubrey de Gray.

What is aging, exactly?
Scientists don't agree on a definition. One theory puts it in terms of oxidation: Just as the oxygen in air causes a cut apple to turn brown or a piece of iron to rust, the oxygen used in human metabolic processes introduces "free radicals" that can damage cells. Over time, those cells die. "In simple language, we don't get old, we rust from oxygen," noted pathologist Harry B. Demopoulos once said. Other scientists see aging through the lens of entropy. According to the second law of thermodynamics, all living and nonliving systems inevitably break down, lose energy, and degrade to a state of inert disorder — death.

Are there ways of avoiding this decline?
So far, there are only ways of slowing it down. One way to lengthen life in at least some creatures is, paradoxically, through near-starvation. Scientists have found that caloric restriction extends the lives of fish, rats, roundworms, and some other species by 30 to 40 percent. No trials have been conducted on humans, and an experiment on rhesus monkeys found that caloric deprivation produced no increases in life span. Still, some true believers called "skinnies" swear by a diet of only 1,200 calories a day — less than half what people usually consume. They're gaunt and hungry, but they're convinced they'll get 10 or 20 extra years of life.

Is there a better alternative?
Several labs around the world are working on replacing failing organs with new ones using stem cell technology. In July, Japanese scientists announced that they had grown the world's first functioning miniature livers from human skin cells. As the science of stem cells and organ building progresses, it may become possible to use a 3D printer to print out new organs on demand. Researchers have already managed to print out a fully beating, three-dimensional mouse heart. Since the cause of most deaths is organ failure, says Anthony Atala, director of the Wake Forest Institute for Regenerative Medicine, finding an easy way to produce replacement organs would "extend life by several decades." Some scientists are even looking for a "cure" for aging itself.

How could aging be cured?
Scientists are focusing on the idea of genetically manipulating mysterious stretches of our DNA called telomeres. Like the plastic tips on the end of shoelaces, telomeres prevent our chromosomes from fraying. But as our cells repeatedly divide over the course of years to renew the body, telomeres become shorter and frayed, until cells can divide no more and grow old and die. If we could somehow manipulate our genes to prevent telomeres from degenerating, then it might be possible to stop aging.

So is immortality a possibility?
Russian multimillionaire Dmitry Itskov certainly thinks so: He's pumped millions into his "2045 Initiative," vowing to cure death within the next three decades. Then there's Google's director of engineering, Ray Kurzweil, who predicts that humans will simply merge with computers, uploading our consciousness and memories and becoming immortal superbeings. So determined is Kurzweil to beat death that he gobbles up to 250 vitamins and other pills a day that he believes will help him live until 2045, when he says immortality will become a reality.

Should we want to live forever?
The idea fills many with dread. Consider a world in which millions of 150-year-olds roamed an already overcrowded planet, placing the environment and our welfare systems under incredible strain. Then there are the unforeseen cultural effects: Would people bother having children if they didn't need to live on through their offspring? Would the institution of marriage crumble under the prospect of 100-year — or eternal — relationships? Ethicist Leon Kass argues that finite life spans are necessary "for treasuring and appreciating all that life brings." But some scientists refuse to accept that death is beyond human control. "It's such a profoundly sad, lonely feeling that I really can't bear it," Kurzweil says. "So I go back to thinking about how I'm not going to die."

The Benjamin Button jellyfish
In 1988, a marine-biology student unwittingly discovered a mysterious sea creature that seemed to debunk the most fundamental law of biology: You are born, you age, and then you die. The tiny species, called Turritopsis dohrnii, appeared to grow younger and younger until it was transformed back to a polyp, its earliest stage of life — hence the nickname "Benjamin Button," after the F. Scott Fitzgerald character who ages in reverse. At that point, the jellyfish would begin its life cycle anew. Today, scientists are working feverishly to discover the immortal jellyfish's secret, believing that an understanding of the inner workings of primitive creatures like jellyfish, sponges, and flatworms could have radical implications for humans. "Once we determine how the jellyfish rejuvenates itself, we should achieve very great things," says marine biologist Shin Kubota. "My opinion is that we will evolve and become immortal ourselves."