A biomarker is not a countdown clock

A biomarker is a measured characteristic that can indicate a biological process or response. Useful markers may help estimate risk in groups or track change, but a reading is not the same thing as a personal expiry date. Measurement error, natural variation, competing risks and the population used to build a model all shape interpretation.

Some biomarkers are clinically established; others are research tools marketed before their meaning is secure. “Biological age” can also refer to several different algorithms that need not agree. The correction is not that measurement is useless. It is that a probabilistic indicator becomes misleading when translated into a literal clock.