A biological-age result can feel like a verdict on how well you are ageing. A number below your age in years may seem reassuring; a higher one may cause concern. The difficulty is knowing how much that number actually says about your health.
‘Biological age’ is not one standard measurement. Some tests use patterns of chemical markings on DNA, known as DNA methylation, to calculate an age-related estimate. These models are called epigenetic clocks. Other models use different inputs, and different clocks may be designed to estimate age, reflect patterns associated with the risk of death or measure the pace of ageing. Those scores are not interchangeable.
The size of the measurement problem can be striking. In a Nature Aging study, repeated laboratory measurements of the same blood sample produced estimates differing by up to nine years for six prominent epigenetic clocks. The sample had not changed; the difference came from the measurement. Nature Aging (source, opens in new tab)
Nature AgingA computational solution for bolstering reliability of epigenetic clocks: Implications for clinical trials and longitudinal trackingOpen source (opens in new tab)
A lower consumer-test score—or a fall between two tests—is therefore not proof that someone has become biologically younger or improved their future health. Interpreting it requires knowing both how reliably the test measures and what its result means for that person. Epigenomics (source, opens in new tab)EpigenomicsFrom population science to the clinic? Limits of epigenetic clocks as personal biomarkersOpen source (opens in new tab) Nature Medicine (source, opens in new tab)
Nature MedicineValidation of biomarkers of agingOpen source (opens in new tab)
The same study showed that measurement consistency could improve. After researchers modified how the clocks calculated their estimates, most repeat results were within 1.5 years of each other. That improvement concerned repeated measurements of the same sample. It did not address every source of variation when someone provides a fresh sample at a later date.
The type of sample matters too. A 2025 study of 83 people compared cheek samples, saliva, dried blood spots and two types of blood-cell samples. Many clock estimates differed significantly between sample types from the same person. Different samples contain distinct cell populations and DNA-methylation patterns, so a score from blood may not be directly comparable with one from saliva or a cheek sample. Aging Cell (source, opens in new tab)Aging CellCross-tissue comparison of epigenetic aging clocks in humansOpen source (opens in new tab)
The Skin and Blood clock showed the greatest overall agreement across sample types among the clocks tested in that study. This underlines why each clock needs to be assessed for its particular use, rather than assuming that all biological-age scores behave alike.
Consistency is only part of the challenge. Reviews find that some ageing biomarkers are linked to later health outcomes across groups of people. But finding a useful pattern across a population does not establish how large a change in one person’s score would be meaningful for their health, or what result should prompt action.
Ageing clocks can help researchers compare groups and assess possible interventions when the tests have been shown to work for those uses. Personal health assessments require further checks: how well a test predicts future ageing-related health outcomes, whether results are comparable and whether it works beyond the setting in which it was developed. There is still no agreed standard for bringing ageing biomarkers into clinical use. For the person receiving the result, the real test is whether the number tells them something dependable about their health.
- Nature Aging · Peer-reviewed studyA computational solution for bolstering reliability of epigenetic clocks: Implications for clinical trials and longitudinal trackingpmc.ncbi.nlm.nih.gov · opens in new tab
- Epigenomics · ReviewFrom population science to the clinic? Limits of epigenetic clocks as personal biomarkerspmc.ncbi.nlm.nih.gov · opens in new tab
- Aging Cell · Peer-reviewed studyCross-tissue comparison of epigenetic aging clocks in humansonlinelibrary.wiley.com · opens in new tab
