A measure of an organism's functional state, distinct from chronological age, often inferred from biomarkers.Biological age is an estimate of how well an organism's body is functioning relative to others of the same calendar age, rather than how many years have passed since birth. Two people both 60 years old by the calendar can differ markedly in biological age: one may have the cardiovascular fitness, kidney function, and immune profile typical of a 50-year-old, the other those of a 70-year-old. The concept exists because chronological age is a crude summary — it counts time, not wear.
The estimate is built from measurable quantities called biomarkers. These can be simple clinical measurements (blood pressure, lung capacity, blood glucose, cholesterol), or high-dimensional molecular data: patterns of chemical tags on DNA that change predictably with age, levels of thousands of proteins or metabolites in blood, or composite scores combining many such signals. A model is trained on large populations to predict chronological age from these markers; the difference between predicted and actual age, sometimes called age acceleration, is then used as the biological age estimate.
Biological age matters because it is meant to be a better predictor of future disease and death than the calendar, and because unlike the calendar it can in principle move. If an intervention — exercise, diet, a drug — lowers biological age, that suggests it is slowing some aspect of aging rather than merely treating a symptom. The concept is also contested: different biomarker panels give different answers for the same person, and it is not settled whether they all track one underlying aging process or several loosely related ones.
The mechanism is statistical rather than physiological. Researchers assemble a large group of people whose chronological ages are known, measure a set of candidate biomarkers in each, and fit a model that predicts age from those measurements as accurately as possible. The model learns which patterns rise or fall with age in the population. When a new person is measured, the model outputs an age estimate; if the estimate exceeds the person's true age, their biology resembles that of an older group.
Different biomarker types capture different processes. DNA methylation clocks read chemical marks added to DNA that shift in a partly predictable way over a lifetime, reflecting cumulative cell division and maintenance history. Proteomic clocks read the abundance of circulating proteins, many of which reflect immune and metabolic activity. Clinical composite clocks combine routine lab values. Because each clock is trained on a particular dataset and outcome, they can disagree, and a clock is only as meaningful as the population it was calibrated on.
Biological age estimates are used in epidemiological studies and clinical trials as secondary endpoints to test whether an intervention affects the pace of aging, and commercial tests offer them directly to consumers, though their regulatory status and clinical usefulness vary.