Guide
What biological age tests can and cannot tell you
7 min read · last reviewed 22 August 2026
Biological age tests promise a seductive number: how old your body really is. The science underneath is genuine — these measures predict mortality better than chronological age across populations. The leap from population statistics to a meaningful personal number is where the promises outrun the evidence.
What these tests actually measure
The two main families are epigenetic clocks, which read chemical marks on DNA that shift with age, and blood-marker composites like PhenoAge, which combine routine laboratory values weighted by how strongly they predicted mortality in large cohorts. Both produce a number on an age scale. Neither measures some underlying true age — each measures its own inputs, calibrated against population death records.
The population-to-person problem
Across thousands of people, a higher biological age reliably predicts higher mortality risk. For one person on one day, the number is noisy: an infection, poor sleep, hydration or a recent hard workout can move blood markers; different clocks frequently disagree about the same person; and test-retest variability can swallow the year-to-year changes people hope to track.
The honest framing: these tests describe how your measured markers compare with population patterns. They do not diagnose anything, and they cannot tell you how long you will live.
The trap to avoid
The commercial ecosystem often pairs a biological age test with the products that supposedly lower it. Be suspicious of that loop: a supplement that shifts a clock reading has demonstrated an effect on the clock's inputs, not on your ageing. No intervention has been shown to extend human lifespan by lowering a biological age score — the score is a surrogate of surrogates.
A reasonable way to use them
Used soberly, a blood-marker age is a compact summary of things worth knowing anyway — glucose, inflammation, kidney and liver markers. If a result flags something, the useful response is the boring one: discuss the underlying markers with a clinician, not chase the composite number with supplements.