Why Two People the Same Age Can Have Completely Different ‘Biological Ages’

Two women can walk into the same clinic on the same day, both 61 years old, both seemingly healthy, and still be aging in opposite directions inside their own cells. That contrast sits at the heart of a new study that tracked the blood chemistry and gene activity of hundreds of women for up to eight years. One person’s genetic activity might be quietly shifting away from markers tied to heart disease, while another’s moves steadily toward it, even though both women share a birth year and a waiting room.

Scientists from King’s College London and several partner institutions followed 335 women over time, drawing blood repeatedly to see how gene activity and blood chemistry changed as they aged. Their work, published in the journal Science, upends the tidy notion that aging follows one predictable path. Broad trends emerged across the group, but individuals often drifted in their own direction, and a few moved against the overall trend.

That distinction matters because most aging research relies on a single blood sample compared against samples from other people at different ages. That kind of snapshot can’t tell scientists whether a 70-year-old’s blood looks different because of how bodies typically age, or because that particular person’s biology is doing something unusual. By following the same women again and again, this study offers a far more personal picture of what aging actually looks like inside a real body.

How the Aging Study Tracked the Same Women for Eight Years

Researchers built what they call the MultiMuTHER study inside the long-running TwinsUK project, which follows British twins throughout their lives. All 335 participants were women, ranging in age from 32 to 80, with a median age of 61 at the start. Each person attended at least three clinic visits between 2009 and 2017, with at least a year between visits and a median gap of six years between a participant’s first and last appointment.

At each visit, researchers drew blood and looked at two things: which genes were switched on in blood cells, and which chemicals, called metabolites, were circulating in the bloodstream. Metabolites include fats, sugars, and hormones that reflect what’s going on inside cells. Combining thousands of gene readings with more than 900 different blood chemicals across multiple visits per person gave researchers an unusually detailed timeline of change, rather than a single frozen moment.

To figure out what counted as a real shift, statisticians compared each woman’s readings across her own visits, then compared those patterns across the whole group. That approach let them separate two different things: trends that showed up across most participants, and quirks that belonged to individual people alone.

What the Data Revealed About Aging in Every Body

Levels of 5,061 genes and 181 metabolites changed measurably over the study period. Genes that lost activity over time were tied to conditions familiar to anyone who has worried about getting older, including diabetes, hardened arteries, and Alzheimer’s disease. Genes that gained activity were fewer in number but included some connected to Parkinson’s and Huntington’s diseases, along with genes involved in basic cell repair.

Averages hide a lot, though. When researchers zoomed in on individual paths, some women tracked the group trend, some barely budged, and others reversed course completely. A number of genes and metabolites showed no consistent group-wide pattern at all, just wildly different paths from one woman to the next, including the brain chemical serotonin and an immune signaling molecule tied to heart aging.

Immune cells told an especially telling story. Researchers estimated activity separately across ten types of immune cells hiding within whole-blood samples. Cells from the “adaptive” immune system, the branch that learns to recognize specific germs over a lifetime, tended to lose gene activity over time, matching the common idea that this part of immunity wears down with age. Natural killer cells, part of the body’s faster, more general-purpose defenses, went the other way and gained activity as participants aged. Aging immunity, then, looks less like an even fade and more like a shift in balance, with the adaptive branch quieting as natural killer cells ramp up, a pattern researchers have long linked to the low-grade inflammation common in older adults.

Timing mattered more than expected, too. About a quarter of genes and metabolites shifted with the seasons, and even more, somewhere between a quarter and nearly 40 percent depending on the measurement, changed depending on the time of day blood was drawn. Genes tied to the body’s internal clock dropped throughout the day, while genes involved in immune activity and fat burning rose later. A single blood test, in other words, can look different depending on when and in what month it’s taken.

Genetics shaped some of these individual paths as well. Suggestive signals turned up at 128 genes where a person’s genetic makeup appeared to influence how their gene activity changed over the years, including genes previously linked to Alzheimer’s risk and to inflammation associated with aging.

One especially concrete real-world finding: blood levels of PFAS chemicals, sometimes called “forever chemicals” because they linger in the environment and the body for so long, dropped over the course of the study. That decline lines up with restrictions the United Kingdom placed on these substances. At the earliest clinic visits, PFAS levels were linked to changes in hundreds of genes and dozens of metabolites, but by the final visits, those links had largely disappeared, fading alongside the exposure itself.

Finally, researchers looked at how gene activity and blood chemistry moved together rather than in isolation. They found more than 100,000 connections between specific genes and specific metabolites, with a handful of “hub” genes and chemicals linked to a large share of the rest. One pattern that emerged strongly tracked with body weight and blood sugar control, tying fat metabolism and inflammation together in a way that lined up with Type 2 diabetes status.

Aging, according to this dataset, isn’t a single dial that turns at the same speed in everyone. It looks more like a bundle of separate, sometimes competing processes, shaped by genetics, the calendar, the clock, and even chemical exposures tied to where and when someone lives. A birthday number alone says very little about what’s actually happening inside a person’s cells. The real story of getting older is written differently in every body, and following each person’s own path, rather than treating everyone’s aging clock as if it ticks in unison, is where some researchers see the best hope of catching disease sooner.

Disclaimer: This article summarizes the findings of a peer-reviewed study for a general audience and is intended for informational purposes only. It is not medical advice. The research describes patterns across a group of participants and cannot predict how any individual will age or diagnose any condition. Readers with questions about their own health should consult a qualified healthcare professional.