Biological age: what the clocks measure, and what they do not

What it means

Biological age is any estimate of ageing based on measurable biology rather than the calendar. The estimate can be built from blood chemistry (the PhenoAge and Levine clocks use nine routine markers), from DNA methylation (the epigenetic clocks), from telomere length, from proteomics, or from a bundle of functional tests.

A biological age below your chronological age is read as "ageing slowly"; above it, as "ageing fast". The number itself matters less than its direction over time, and different clocks frequently disagree with each other by several years for the same person.

Why it matters for longevity

If biological age could be measured well, it would be the single most useful number in longevity: a way to test whether an intervention is working in years rather than waiting decades for outcomes. The clocks are getting closer to that, but they are not there yet, and treating a clock reading as a verdict is the most common mistake in the field.

What the evidence says

The second-generation clocks do predict outcomes. GrimAge and PhenoAge each predict all-cause mortality, cardiovascular disease and functional decline better than chronological age alone, across multiple cohorts. DunedinPACE, which measures the rate of ageing rather than the accumulated amount, tracks with cognitive and physical decline in the Dunedin birth cohort.

What the clocks have not yet shown is that moving the clock moves the outcome. The CALERIE trial of caloric-restriction slowed DunedinPACE by about 2–3%; whether that translates into fewer disease years is unknown. Test-retest noise on many commercial clocks is three to five years, which swamps the effect of most interventions over a year.

How to measure or use it

• The cheapest estimate is a blood-chemistry clock (PhenoAge) computed from a routine panel: albumin, creatinine, glucose, CRP, lymphocyte %, mean cell volume, red-cell distribution width, alkaline phosphatase, white-cell count.
• A methylation clock costs more; if you use one, use the same vendor every time and read only the trend over two or more years.
• Pair any clock with the functional markers — VO2max, grip — which move faster and have harder evidence.

Related

• Biomarkers of ageing — the panel behind the clocks
• What is healthspan? — what the number is trying to predict
• Baseline labs — what to draw first