The Spanish firm Grifols helped set off a kerfuffle last year when it, along with other firms, offered nearly double the going price for blood donations for a COVID-19 treatment trial. Brigham Young University in Idaho had to threaten some enterprising students with suspension to keep them from intentionally trying to contract COVID-19. The trial failed, however, and now the Barcelona-based firm is hoping to extract something far more valuable from the plasma of young volunteers: a set of microscopic molecules that could reverse the process of aging itself. Earlier this year, Grifols closed on a $146 million-deal to buy...