Available studies show that the average difference between biological and chronological age in large populations is small, with a spread of approximately 1 to 2 years, but the studies do not name an absolute upper limit. Biological age predicts disease risk and death better than birth year, but which number emerges depends strongly on the measurement method used. The research is still at an early enough stage that consumers can take two different tests on the same day and still get a different number.
Chronological age is simply the number of years you have lived, whereas biological age measures how old your body looks physiologically. A large Chinese cohort study (China Kadoorie Biobank, more than 12,000 participants) using the so-called KDM method, which combines 16 physical and 9 biochemical markers, found that the spread around the average difference between biological and chronological age was approximately 1.2 to 1.6 years (expressed as a standard deviation). The average deviation per group was small, just a few tenths of a year, but individual outliers can be larger1. Exact upper limits, such as 'a maximum of 10 years', are not given in the available studies.
Multiple review studies do confirm that people can age at considerably different rates, but the precise upper limit of that difference depends strongly on which measurement you use and which population is being studied2,3,4. In other words, the answer to 'how large can the difference be?' is not a single number, but a range that varies by measurement method and by individual.
Furthermore, 'the' biological age is not a single number. Within one and the same person, different organs, such as the liver, kidneys, immune system and metabolic system, can each age at their own pace5. Your biological age based on blood values can therefore differ from your biological age based on, say, a liver or kidney measurement. This makes comparisons between studies and measurement methods difficult.
What biological age measurements can do is predict the risk of disease and death more accurately than chronological age alone. The KDM method predicted death in an NHANES cohort study of 9,389 participants with 18 years of follow-up considerably better than birth year; once biological age was included in the model, chronological age added hardly anything further6. The GOLD BioAge model, which works with blood values, proteins and metabolic markers, showed a similar picture: people who were biologically older than their chronological age had a significantly higher chance of chronic disease and earlier death in both the NHANES and UK Biobank data7.
Epigenetic clocks, which work with methylation patterns in DNA (well-known examples are Horvath, Hannum, PhenoAge and GrimAge), are a different approach and sometimes produce different outcomes than methods based on blood values. The clocks do correlate with disease and death, but discrepancies between clocks are a recognised weakness of this field, and high-quality comparative studies are still lacking3. For a consumer, this means: two different biological age tests can give a different number, even if they are taken on the same day.
All claims are based on observational and associative studies; no RCTs. Most studies measure biological age via blood values or epigenetic markers, not via direct physical outcomes. The China Kadoorie Biobank (PMID 36758480) and NHANES (PMID 23213031) are the largest populations with concrete figures.