For generations, the question of whether a treatment truly slows human aging has demanded the most unforgiving of timekeepers: a human lifetime. A new meta-analysis of 51 longevity studies, published in Nature Medicine, brings science closer to a more practical answer, identifying which epigenetic clocks — biological measures of aging inscribed in our DNA — respond most reliably when people take drugs or change how they live. The work does not yet prove that these clocks measure aging itself, but it narrows the search, pointing researchers toward the instruments most worthy of deeper trust.