The researchers kept the exchange going until each older mouse had about half of its own blood and half of a younger mouse’s blood, and vice versa. After a month, they tested each mouse’s organ systems, including liver and brain cell development, as well as muscle tissue repair. While the older mice didn’t significantly improve at all, the younger mice actually declined in health. Each organ system declined, with particularly profound effects in brain cell development. In other words, the young blood did nothing for the older mice, but the older blood had pretty significant detrimental effects on the younger mice’s health.