Ageing science

Biological age vs chronological age: what the tests measure, and what they cannot prove

Chronological age is a fact. 'Biological age' is a family of statistical estimates, each built for a different purpose. Why two tests can disagree, and why a younger result is not proof of a longer life.

By The SnapLongevity Team10 October 20266 min read

The same calendar age can look and feel different from person to person. Measuring that difference reliably is harder than it sounds.

Two people born on the same day have the same chronological age for the rest of their lives. They do not age in the same way: one may develop heart disease at 60 while the other runs at 80. 'Biological age' is the attempt to put a number on that difference. The idea is sound. The difficulty is that there is no single biological age to measure, only a growing set of estimates, each defined by the data it was built on.

In plain English

Chronological age: Time since birth. Known exactly, and the basis of every life table on this site.
Biological age: A statistical estimate, expressed in years, of how far a person's physiology or molecular markers resemble those typical of an older or younger age.Why it matters: It is a model output, so its meaning depends on what the model was trained to predict.
Age acceleration: The difference between an estimated biological age and chronological age, often after statistical adjustment.

Epigenetic clocks

Most commercial and research claims about biological age rest on DNA methylation — chemical tags on DNA that change with age. In 2013 Steve Horvath published a clock built from 353 methylation sites across 51 tissues and cell types that estimates chronological age closely. These first-generation clocks were trained to predict calendar age, so the part they fail to predict is what gets interpreted as biological ageing.

Second-generation clocks changed the target. PhenoAge (2018) was trained on a composite of clinical blood markers linked to mortality; GrimAge (2019) combines methylation-based stand-ins for plasma proteins and smoking history, calibrated on time to death. DunedinPACE (2022) does something different again: it estimates the pace of ageing, derived from how quickly 19 indicators of organ-system integrity declined over two decades in one New Zealand birth cohort.

What does a 'biological age' measure?

One calendar age, several different kinds of estimate

Chronological age

Years since birth

Calibrated on The calendar

Produces Exact, the same for everyone born on the same day

First-generation epigenetic clocks

Horvath (2013) multi-tissue clock

Calibrated on Chronological age

Produces An age estimate; the gap from calendar age is the 'acceleration'

Second-generation epigenetic clocks

PhenoAge (2018), GrimAge (2019)

Calibrated on Mortality and clinical markers

Produces An age-scaled risk score

Pace-of-aging measures

DunedinPACE (2022)

Calibrated on Rate of decline across organ systems

Produces Years of biological change per calendar year

Clinical biomarker composites

Composites of markers such as blood pressure, lipids, kidney and liver function

Calibrated on Routine blood and physiological tests

Produces An age-scaled summary of measured physiology

Simplified classification drawn from the cited papers. Each method is calibrated against a different target, which is one reason two tests on the same person can give different results. In the Dunedin Study, eleven such measures showed low agreement with one another (Belsky et al., 2018).

Clinical biomarker-based estimates

Another approach combines routine clinical measurements — blood pressure, lung function, blood lipids, glucose, kidney and liver markers — into a single age-scaled score. These composites use tests that doctors already understand, but they are still models: the result depends on which markers are included and on the population used to calibrate them.

Why different tests can disagree

When researchers applied eleven measures of biological ageing — telomere length, three epigenetic clocks and several biomarker composites — to the same 964 middle-aged people in the Dunedin Study, they found low agreement between them. Some were related to physical functioning and facial ageing, with modest effect sizes. Their conclusion was that the approaches may not measure the same aspects of ageing.

Predictive validation

A measure predicts later disease, disability or death in a population better than chronological age alone. Several second-generation clocks meet this standard in research cohorts.

Clinical usefulness

A measure changes decisions or outcomes for an individual patient. No biological-age test has established this for routine care.

A population association can be real and still give an individual result with substantial measurement error and little guidance on what to do.

What 'reversal' studies show — and do not show

In 2019, a small uncontrolled study of a thymus-regeneration drug combination reported that participants' epigenetic age was about 1.5 years lower than at baseline after one year. The authors described this as epigenetic age reversal. The study had no randomised control group, and it measured a biomarker, not lifespan.

A stronger design came from CALERIE, a randomised trial in which 220 adults without obesity were assigned to 25% calorie restriction or their usual diet for two years. A later analysis found that restriction slowed DunedinPACE, but did not significantly change PhenoAge or GrimAge, and the effects were small. The authors noted that a conclusive test would need long-term follow-up of disease and mortality.

Where this leaves the term

Biological ageing is real, and people of the same age differ in it.
Current biological-age measures are research tools that capture different aspects of that variation.
Results from different tests are not interchangeable and can disagree for the same person.
No measure has yet been shown to be a validated substitute for lifespan in human trials.

Population life tables, by contrast, use chronological age only. For what that tells you at each age, see why life expectancy changes with age.