Parabiosis:
Mr. Thiel is focused on ways to prolong life. He was intrigued by parabiosis, a blood regeneration trial in which people over 35 would receive transfusions from people aged 16 to 25 — an experiment that Anne Rice gave a thumbs up to. [The New York Times]
Similar experiments have been performed on mice, both from young mice to old mice, and young humans to old mice (not a typo!), with positive results.