If you transplant fecal microbes from old to young rats, you age the young rats. We have solid evidence that blood transfusions from old mammals to young mammals will age the young recipient (Nature, 2016). The other direction, evidently more useful, is achievable in vitro, but harder to achieve in actual organisms. In what might be a breakthrough, Horvath et al. report system-wide rejuventation in rats using plasma (blood) transfusion: the treatment more than halved the epigenetic ages of blood, heart, and liver tissue. Details are missing and we should reserve judgement. Horvath is a well regarded scientist from UCLA.