Longevity 101
Biological Age vs Chronological Age:
What's the Difference?
Your chronological age is the number of years since you were born. Your biological age is an estimate of how old your body seems, based on how well its systems are working. The two can differ widely. In the Dunedin Study in New Zealand, researchers measured 954 people who were all 38 years old. Their estimated biological ages ranged from 28 to 61, and some were ageing at almost three years of biological change per calendar year while others barely aged at all. Those ageing faster had poorer balance and strength, showed signs of cognitive decline and were even rated as looking older by strangers. Different organs can also age at different rates: in one large study, almost 20% of people had one organ ageing noticeably faster than expected. Biological age is shaped far more by lifestyle and environment than by genes: in a study of nearly 500,000 people, environmental factors explained 17% of the variation in risk of death, compared with less than 2% for genetic predisposition. Importantly, biological age can change, temporarily rising with severe stress or illness and slowing with healthy habits. This article explains what biological age is, what shapes it and why it matters.
Key numbers
| Finding | Detail |
|---|---|
| Same age, different bodies (Dunedin Study, 954 people aged 38) | Biological ages ranged from 28 to 61 |
| Pace of ageing (same study) | From near zero to nearly 3 years of biological change per calendar year |
| Organ ageing (5,600+ people) | Almost 20% had one organ ageing faster than expected |
| Environment vs genes (nearly 500,000 people) | Environment and lifestyle explained 17% of variation in death risk; genetic predisposition less than 2% |
| Heritability of lifespan | About 7% or less |
Chronological age vs biological age
- Chronological age is fixed: it increases by exactly one year every year.
- Biological age reflects the condition of your body: your heart and blood vessels, metabolism, muscles, lungs, kidneys, immune system and brain. It can be higher or lower than your chronological age, and it can change.
Biological age is estimated rather than measured directly. Different methods, such as blood test algorithms, epigenetic clocks, fitness tests and body composition, capture different aspects of ageing, so they don't always agree (see How Is Biological Age Calculated?).
What shapes biological age
- Genes: play a role, but a smaller one than most people think. A study of family trees covering about 400 million people estimated that genes explain about 7% or less of the variation in lifespan (see The Genetics of Longevity).
- Lifestyle: smoking, physical activity, diet, sleep, alcohol and body weight.
- Environment and circumstances: socioeconomic conditions, education, air pollution and early-life factors.
- Health conditions: high blood pressure, diabetes, chronic inflammation and other diseases.
- Short-term stress: severe illness, surgery or major stress can raise biological age temporarily.
A 2025 Oxford study of nearly 500,000 people found that environmental and lifestyle factors explained 17% of the variation in risk of death, compared with less than 2% for genetic predisposition, with smoking, socioeconomic status, physical activity and living conditions having the greatest impact.
What the research shows
People of the same age age at very different rates. In a 2015 study in PNAS, researchers from the Dunedin Study, which has followed everyone born in one New Zealand city in 1972-73 since birth, combined 10 blood and health measures to estimate biological age at 38. Biological ages ranged from 28 to 61. Using 18 measures of heart, metabolic, immune, kidney, liver, lung and dental health tracked from age 26 to 38, they found that some people were ageing at nearly three years per calendar year, while others showed almost no change. Faster agers had worse balance, strength and coordination, lower cognitive scores, signs of small blood vessel changes in the eye, reported poorer health and were rated as looking older.
Organs age at different rates. Using thousands of blood proteins, Stanford researchers estimated the biological age of 11 organs in more than 5,600 people. Almost 20% had one organ ageing faster than expected, and those with fast-ageing hearts had more than double the risk of heart failure (see Organ Ageing Clocks).
Biological age can change. Biological age rose sharply after emergency surgery, during pregnancy and with severe COVID-19, then returned towards baseline with recovery. In the CALERIE trial, moderate calorie restriction slowed the pace of ageing by 2-3% (see Can You Reverse Your Biological Age?).
Why this matters for longevity
Biological age helps explain why two people of the same age can have very different health, energy and risks. It shifts the focus from the number on your birth certificate, which you can't change, to the condition of your body, which you can influence. It also reminds us that ageing starts long before old age: differences between people are already visible in their 30s, decades before most chronic diseases appear.
Practical notes
Think of biological age as a concept rather than a single number. Different tests give different answers, and no test is perfect. What matters most is the direction of travel: improving the measures that reflect how your body is ageing, such as blood pressure, blood sugar, cholesterol, fitness, strength, body composition and sleep. The key numbers to track are covered in Your Key Longevity Numbers. The habits that slow biological ageing are the same ones that protect against chronic disease: not smoking, regular exercise, a healthy diet, good sleep and managing stress (see From Hallmarks to Habits). Our Longevity Doctors can help you understand where you stand, starting with the free longevity assessment.
- Belsky DW, et al. Quantification of biological aging in young adults. Proceedings of the National Academy of Sciences, 2015;112(30):E4104-E4110.
- Oh HSH, et al. Organ aging signatures in the plasma proteome track health and disease. Nature, 2023;624(7990):164-172.
- Argentieri MA, et al. Integrating the environmental and genetic architectures of aging and mortality. Nature Medicine, 2025;31(4):1016-1025.
- Ruby JG, et al. Estimates of the heritability of human longevity are substantially inflated due to assortative mating. Genetics, 2018;210(3):1109-1124.
- Poganik JR, et al. Biological age is increased by stress and restored upon recovery. Cell Metabolism, 2023;35(5):807-820.
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