The quest to live forever has fascinated humans for millennia. The Epic of Gilgamesh , composed about 4,000 years ago, follows a king who searches the world for a plant that can restore youth, only to lose the plant to a thieving snake. The (likely apocryphal) story of Juan Ponce de León, who is said to have embarked on a search for the Fountain of Youth in the early 16th century, refuses to die—unlike its protagonist, who was killed along his journey. Today’s longevity-medicine movement is