Around this time last year I was attending an aging conference in Manchester, listening to a talk about fly aging, when my phone started pinging. News outlets were reporting that a hot mic had caught Russia’s and China’s leaders discussing the possibility of living forever. 'With the developments of biotechnology, human organs can be continuously transplanted, and people can live younger and younger, and even achieve immortality,' Russia’s Vladimir Putin reportedly told China’s Xi Jinping. He seemed to have been referring to the 'replacement' theory of longevity, which has been supported by multiple experiments that involved physically stitching young mice to old ones.

Something about the young blood rejuvenated the old mice. Perhaps young organs could rejuvenate world leaders in their 70s, too. Unfortunately for Putin, new research pours a little cold water on this idea. Studies on transplanted hearts in both mice and humans suggest that new hearts soon adopt the biological age of the recipient, no matter how young they were to begin with.

To find out, Jesse Poganik at Brigham and Women’s Hospital in Boston and his colleagues performed a set of heart transplants in mice. The team used a trio of 'aging clocks'—molecular tools used to estimate the biological ages of tissues and whole organisms—to assess whether the additional hearts affected the mice in any way. When analyzed between four and six months after surgery, the transplanted hearts had adopted the biological age of the recipients. The finding was backed up by data from human heart transplants.