Blood plasma from young people has been found to rejuvenate old mice, improving their memory, cognition, and physical activity. The method has the potential to be developed into a treatment for people, says Sakura Minami of Alkahest, the company behind the work.
The paper was presented at the Society for Neuroscience annual meeting in San Diego and has not been published yet. I will post the abstract when it becomes available.
By the way, this idea was the basis for the 1942 film The Corpse Vanishes.