This week Im writing from Manchester, where Ive been attendinga conference on aging. Wednesday was full of talks and presentations by scientists who are trying to understand the nitty-gritty of agingall the way down to the molecular level. Once we can understand the complex biology of aging, we should be able to slow or prevent the onset of age-related diseases, they hope.Then my editor forwarded me a video of the leaders of Russia and China talking about immortality. These days at 70 years old you are still a child, Chinas Xi Jinping, 72, was translated as saying, according to footage livestreamed by CCTV tomultiplemedia outlets.With the developments of biotechnology, human organs can be continuously transplanted, and people can live younger and younger, and even achieve immortality, Russias Vladimir Putin, also 72, is reported to have replied.SERGEI BOBYLEV, SPUTNIK, KREMLIN POOL PHOTO VIA APTheres a striking contrast between that radical vision and the incremental longevity science presented at the meeting. Repeated rounds of organ transplantation surgery arent likely to help anyone radically extend their lifespan anytime soon.First, back to Putins proposal: the idea of continually replacing aged organs to stay young. Its a simplistic way to think about aging. After all, aging is so complicated that researchers cant agree on what causes it, why it occurs, or even how to define it, let alone treat it.Having said that, there may be some merit to the idea of repairing worn-out body parts with biological or synthetic replacements. Replacement therapiesincluding bioengineered organsare being developed by multiple research teams. Some have already been tested in people. This week, lets take a look at the idea of replacement therapies.No one fully understands why our organs start to fail with age. On the face of it, replacing them seems like a good idea. After all, we already know how to do organ transplants. Theyve been a part of medicine since the 1950s and have been used tosave hundreds of thousands of lives in the US alone.And replacing old organs with young ones might have more broadly beneficial effects. When a young mouse is stitched to an old one, the older mouse benefits from the arrangement, and its health seems to improve.The problem is that we dont really know why. We dont know what it is about young body tissues that makes them health-promoting. We dont know how long these effects might last in a person. We dont know how different organ transplants will compare, either. Might a young heart be more beneficial than a young liver? No one knows.And thats before you consider the practicalities of organ transplantation. There is already a shortage of donor organsthousands of people die on waiting lists. Transplantation requires major surgery and, typically, a lifetime of prescription drugs that damp down the immune system,leaving a person more susceptible to certain infections and diseases.So the idea of repeated organ transplantations shouldnt really be a particularly appealing one. I dont think thats going to happen anytime soon, says Jesse Poganik, who studies aging at Brigham and Womens Hospital in Boston and is also in Manchester for the meeting.Poganik has been collaborating with transplant surgeons in his own research. The surgeries are good, but theyre not simple, he tells me. And they come with real risks. His own 24-year-old cousin developed a form of cancer after a liver and heart transplant. She died a few weeks ago, he says.So when it comes to replacing worn-out organs, scientists are looking for both biological and synthetic alternatives.Weve been replacing body parts for centuries. Wooden toes were used as far back as the 15th century. Joint replacements have been around for more than a hundred years. And major innovations over the last 70 years have given us devices like pacemakers, hearing aids, brain implants, and artificial hearts.Scientists are exploring other ways to make tissues and organs, too. There are different approaches here, but they include everything from injecting stem cells to seeding scaffolds with cells in a lab.In 1999, researchers used volunteers own cells to seed bladder-shaped collagen scaffolds. The resulting bioengineered bladders went on to be transplanted into seven people inan initial trial.Now scientists are working on more complicated organs. Jean Hbert, a program manager at the US governments Advanced Research Projects Agency for Health, has been exploring ways to gradually replace the cells in a persons brain. The idea is that, eventually, the recipient will end up with a young brain.Hbertshowed my colleague Antonio Regalado how, in his early experiments, he removed parts of mices brains and replaced them with embryonic stem cells. That work seems a world away from the biochemical studies being presented at the British Society for Research on Ageing annual meeting in Manchester, where I am now.On Wednesday, one scientist described how hed been testing potential longevity drugs on the tiny nematode worm C. elegans. These worms live for only about 15 to 40 days, and his team can perform tens of thousands of experiments with them. About 40% of the drugs that extend lifespan in C. elegans also help mice live longer, he told us.To me, thats not an amazing hit rate. And we dont know how many of those drugs will work in people. Probably less than 40% of that 40%.Other scientists presented work on chemical reactions happening at the cellular level. It was deep, basic science, and my takeaway was that theres a lot aging researchers still dont fully understand.It will take yearsif not decadesto get the full picture of aging at the molecular level. And if we rely on a series of experiments in worms, and then mice, and then humans, were unlikely to make progress for a really long time. In that context, the idea of replacement therapy feels like a shortcut.Replacement is a really exciting avenue because you dont have to understand the biology of aging as much, says Sierra Lore, who studies aging at the University of Copenhagen in Denmark and the Buck Institute for Research on Aging in Novato, California.Lore says she started her research career studying aging at the molecular level, but she soon changed course. She now plans to focus her attention on replacement therapies. I very quickly realized were decades away [from understanding the molecular processes that underlie aging], she says. Why dont we just take what we already knowreplacementand try to understand and apply it better?So perhaps Putins straightforward approach to delaying aging holds some merit. Whether it will grant him immortality is another matter.This article first appeared in The Checkup,MIT Technology Reviewsweekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first,sign up here.