Turns out that most 10,000-year-olds have learned a thing or two about fitness: young blood, researchers now confirm, can make old bodies young again. The New York Times reports on the research, conducted at Stanford Medical School and at Harvard, which literally joined the blood vessels of young and old mice for five weeks. The researchers first noted that the old mice had grown new livers as quickly as young mice do, and also that the young mice attached to old mice seemed to age more quickly than they should. The researchers found that a protein, called GDF11, was abundant in young mice and scarce in old ones. Wait, there’s more. Other research indicates that young blood stimulates the growth of neurons in the hippocampus of brains in old mice, and that GDF11 revived stem cells in old muscles, making old mice stronger and increasing their endurance. Read more in the New York Times.