Fascinating stuff, discovered very recently at UCSF and two other places, mostly independently:
A protein released by blood platelets, PF4, improves cognition in elderly mice. It declines as we age (in both mice and people). It may be the reason why young blood improves cognition and reduces frailty in aging mice, when blood vessels are conjoined to younger mice.
Both Calorie restriction and exercise also increase PF4 levels in the blood (of humans).
Read:
https://www.sciencenews.org/article/platelet-pf4-aging-brain-longevity
-- Saul