There's data from clinical trials. Before doing experiments with humans a company has to make the experiments in mice (or other animals). Any clinical trial of a drug that fails can be seen as a trial where a hypothesis failed to generalize. If I remember right the FDA does release some of the relevant data.
In experiments performed on mice, blood transfusions from young mice reversed age-related markers in older mice. The protein involved is identical in humans.
http://mic.com/articles/88851/harvard-scientists-may-have-just-unlocked-the-secret-to-staying-young-forever