I vaguely recall that it's the blood itself which has a big impact — to the point where blood transplants from younger blood could (temporarily) "rejuvenate" an older metabolism.
Farming young humans to expand the lifespan of old rich ones has a little bit of a je ne sais quoi though.
Just kill the donors as soon as possible to end their suffering, then /s
I don’t know who that is and I’m sure he couldn’t have transfused “blood”. But parabiosis experiments in mice uncovered that certain components present in young blood (like PF4) can be beneficial.