Aging doesn't look the same for everyone at the molecular level — it's 'individualized and context-dependent'

People who are the exact same chronological age can undergo very different molecular changes as they grow older, a new study finds.

"Molecular aging is dynamic and unique to each person," said study co-author Julia El-Sayed Moustafa , a computational genomics researcher at King's College London. Even if you and your friend were born the same day, the activity of some of your genes might ramp up while the same genes in your friend dial down. The same is true for metabolites, which are small molecules produced during the body's chemical processes essential for health.

Genetics, environmental exposures and daily biological rhythms may help to explain some of those differences, the researchers say.

The findings, published Thursday (Sept. 3) in the journal Science , may complicate efforts to capture aging with a single metric. For example, "biological age" scores estimate the body's true cellular and physical health, judging how much "older" it is than your chronological age. Scientists have developed " aging clocks " that use molecular markers, such as chemical tags on DNA, to estimate biological age.

But this new study suggests that "biological aging is less like a fixed score and more like a changing set of molecular trajectories," said Raghav Sehgal , an associate research scientist at the Yale School of Medicine who was not involved in the new research. "No single number can fully capture the many systems that age at different rates," he told Live Science in an email.

In the study, researchers followed 335 women ages 32 to 80 from the long-running TwinsUK cohort for eight years. The cohort includes identical and fraternal twin pairs, along with detailed health and biological data gathered to investigate how genetics and environment can influence health.

Each woman had at least three clinic visits between 2009 and 2017, with a median of six years between their first and last visits. At each visit, researchers took blood samples and measured gene activity and levels of metabolites.

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