People who are the same age can experience very different molecular changes as they age, according to a new study. Researchers found that the way genes and small molecules called metabolites change over time varies from person to person, even among those born on the same day. "Molecular aging is dynamic and unique to each person," said Julia El-Sayed Moustafa, a computational genomics researcher at King's College London. This means that while one person's genes might become more active with age, another's might become less active, and the same goes for metabolites, which are essential for bodily functions. The study, published in the journal Science, suggests that factors like genetics, environment, and daily rhythms influence these molecular changes. This complexity challenges the idea of measuring aging with a single number, such as "biological age," which tries to estimate how old a person's body is compared to their actual age. Instead, the research indicates that aging is more like a collection of changing processes, not a fixed score. "No single number can fully capture the many systems that age at different rates," said Raghav Sehgal, a researcher at Yale School of Medicine not involved in the study. The study followed 335 women aged 32 to 80 from the TwinsUK cohort, a long-term research group that includes twins and detailed health data. Researchers collected blood samples and measured gene activity and metabolite levels during multiple visits between 2009 and 2017. This approach allowed them to observe how these biological markers changed over time within the same person, rather than just comparing different people at one point in time. Out of more than 16,000 genes and 915 metabolites analyzed, over 5,000 genes and 180 metabolites showed significant changes. Many of these genes were involved in processes linked to immune function, metabolism, and age-related diseases. The study also found that while most people showed similar trends in gene activity and metabolite levels, some individuals had opposite patterns. Identical twins had more similar gene-expression patterns than fraternal twins, pointing to a strong genetic influence. However, the researchers also noted that molecular changes varied depending on the time of day and season. Some genes and metabolites showed seasonal shifts, and others followed the body’s internal 24-hour clock. Additionally, the study found a decline in levels of certain synthetic chemicals known as PFAS, likely due to regulations in the U.K. These chemicals were linked to changes in some genes and metabolites, though the study could not prove a direct cause-and-effect relationship. The research highlights the complexity of aging and how interconnected biological processes are. While the findings suggest that aging is not entirely random, they also show that it is highly individualized and influenced by many factors. Future studies will need to include larger and more diverse groups to confirm these patterns. Researchers plan to track molecular changes over a longer period, and some experts suggest that future aging assessments might combine general biological age scores with more personalized measures of health.