How reliable are the biological age tests you can buy online?

Epigenetic clocks measure ageing patterns in your DNA and, at the population level, produce a surprisingly accurate estimate of a person's age. They work by reading chemical tags on the DNA (methylation), which change systematically as you get older. That is the scientific foundation underlying most online biological age tests.

Across large groups of people the associations are impressive. The GrimAge clock, developed on the basis of blood proteins and smoking, proved able to predict, in thousands of people, who would die sooner or develop cardiovascular disease or cancer. That is a genuinely strong signal. Other clocks show that people who look 'older' in their DNA also more often have a smaller hippocampal volume, a brain region involved in memory.

For you as an individual, however, the step from 'works in large groups' to 'tells me something useful' is a large one. There is currently no consensus on what a reliable ageing test should measure, how the outcome should be validated, or what a higher or lower score means in your specific case. Clinical standards are largely absent.

There is nonetheless reason to take the results seriously as a lifestyle mirror. Early studies suggest that caloric restriction, a plant-based diet, physical exercise and certain medications can lower the test outcome. But rigorous confirmation of this is still lacking. Whether taking the test seriously actually reduces your health risk is a different question from whether the measurement is statistically sound.

Finally: many authors of relevant studies have commercial interests in ageing companies. CheekAge, a cheek-swab test that shows reproducibility across repeated measurements, was published by employees of the company that sells the test. That is not a reason to ignore the findings, but you are better off reading the results with that knowledge in mind.