For most of medical history, ageing has been treated as a fixed backdrop rather than something we can change. Yet it remains the single largest risk factor behind the chronic diseases that cause most deaths worldwide. The central question now is whether ageing can be measured accurately, separated from simple chronological time, and slowed or reversed without causing more harm than good. This is difficult because ageing does not happen uniformly. It moves at different speeds across organs, tissues, and even individual cells, and testing whether something works in humans can take decades.