Researchers connected the blood vessels of young and old mice, so that two animals shared a blood supply. They found that, following an injury, the older mice saw greater improvements in the repair of muscle and bone compared with older mice not connected to younger mice. Older mice that were exposed to young blood saw an increase in the number of new brain cells in a part of the brain linked with memory. Injecting blood plasma from young mice into older mice for three weeks improved their learning and memory. The older mice that were given young blood were more easily able to find a hidden platform in a water maze, compared with mice given older blood. Young blood contains more of the compounds that are involved in the repair and maintenance of tissues than older blood does.
http://www.delta-optimist.com/opinion/blogs/deltabc-1.983313/blog-old-mouse-with-young-blood-1.2020819