Biological Age vs Chronological Age: How It's Measured

Biological age is an estimate of how old your body seems from the inside, based on markers such as patterns on your DNA or standard blood tests, while chronological age is simply the years since you were born. Two people born the same year can have different biological ages, and research links an older biological age to a higher risk of poor health later on. This guide explains what biological age is, how it is measured, how accurate the tests and calculators are, what affects it, and what the research says about slowing it.

It is educational, not medical advice.

Key Takeaways

Most tests use epigenetic clocks: one early clock estimated age from DNA methylation using about 8,000 samples covering 51 healthy tissues and cell types (Horvath, 2013).
Newer clocks predict health, not just age: clocks such as PhenoAge and GrimAge were built to predict lifespan and healthspan (Levine et al., 2018; Lu et al., 2019).
Results can be noisy: technical noise produced differences of up to 9 years between repeat measurements of the same sample for six well-known clocks (Higgins-Chen et al., 2022).
Lifestyle shows up: in over 4,500 adults, a younger epigenetic age was linked with eating fish and plenty of fruit and vegetables, more education and a healthy weight (Quach et al., 2017).
It may be modifiable, a little: two years of eating 25% fewer calories modestly slowed one measure of the pace of aging in a randomized trial, but did not change other clocks (Waziry et al., 2023).

Featured in this article

NAD+ Nasal Spray

NAD+ and alkaline water, nothing else, in three strengths.

Shop NAD+ Nasal Spray

Third-party tested ingredients. Vegan, non-GMO, made in the USA.

Biological age: an estimate of how old your body seems, measured by epigenetic clocks; lab results can vary by years, and fish, plants, activity and a healthy weight are linked with a younger age.

What Is Biological Age?

Your chronological age is fixed: it goes up by one every birthday. Your biological age is an estimate of how far your body has aged, measured from signals inside your cells and blood. If your biological age is lower than your chronological age, your body looks younger than average for your years on that measure; if it is higher, it looks older.

Researchers care about it because people of the same age age at different rates, and measures of biological age aim to catch that difference years before it shows up as health problems.

Biological Age vs Chronological Age

	Chronological age	Biological age
What it is	Years since you were born	An estimate of how old your body seems
How it is found	Your birth date	DNA methylation, blood markers or other measures
Can it change?	Only upward, one year at a time	It can run faster or slower than the calendar
How exact	Exact	An estimate, with measurement error

How Is Biological Age Measured?

There is no single test. The main approaches are:

First-generation epigenetic clocks. DNA methylation is a set of chemical tags on DNA that shift with age. Clocks built from these tags estimate age closely; one used about 8,000 samples from 51 healthy tissues and cell types (Horvath, 2013), and another was built from blood methylation profiles to compare aging rates between people (Hannum et al., 2013).
Health-trained clocks. PhenoAge was built from DNA methylation linked to a composite of clinical measures, and predicted outcomes such as mortality and physical functioning better than earlier clocks (Levine et al., 2018). GrimAge combines methylation-based estimates of seven blood proteins and of smoking history, and strongly predicted time to death and heart disease (Lu et al., 2019).
Pace of aging. Instead of an age in years, DunedinPACE estimates how fast you are aging right now, from one blood sample. It was built by tracking 19 measures of organ health in the same people over two decades (Belsky et al., 2022).
Online calculators. Free biological age calculators usually estimate from questions about your habits, weight and health. They can be a prompt to look at your habits, but they are not a lab measure.

How Accurate Are Biological Age Tests?

Lab tests are much more informative than questionnaires, but they are not exact. When the same samples were measured twice, technical noise alone moved results by up to 9 years for six prominent clocks; a refined method brought most repeats within 1.5 years (Higgins-Chen et al., 2022). Different clocks can also disagree with each other, because they were built to capture different things.

Practical tips if you test:

Use the same test and clock each time, so results are comparable.
Look at the trend over months or years rather than one number.
Do not change medicines or make big decisions from one result; talk it through with a doctor.

What Affects Biological Age?

In an analysis of 4,173 women from the Women's Health Initiative and 402 adults from an Italian cohort, a younger epigenetic age was linked with eating fish, higher blood carotenoids (a sign of eating fruit and vegetables), more education and moderate alcohol intake, while a higher BMI was linked with an older epigenetic age (Quach et al., 2017). The authors concluded the results support a plant-rich diet with lean meats, physical activity and education. This was an observational study, so it shows links rather than proof of cause, and it is not a reason to start drinking.

Smoking matters too: GrimAge includes a methylation-based estimate of smoking history because it adds to the clock's ability to predict lifespan (Lu et al., 2019). Stress and sleep are also part of the picture; our guides to stress and aging and how much deep sleep you need cover them.

Oxidative damage is one of the processes that builds with age; our guide to oxidative stress explains what raises and lowers it.

Can You Lower Your Biological Age?

The best human test so far is the CALERIE trial, in which 220 healthy adults without obesity were randomly assigned to eat 25% fewer calories or to eat as usual for two years. Calorie restriction slowed the pace of aging measured by DunedinPACE, but did not significantly change biological age on other clocks, including PhenoAge and GrimAge, and the effects were small (Waziry et al., 2023). Our guide to fasting and longevity covers this area in more detail.

Beyond that, the habits linked with a younger biological age are the familiar ones: not smoking, staying active, keeping a healthy weight, eating plenty of plants and fish, sleeping well and managing stress. They are worth doing for their own sake, whatever a clock says. Our anti-aging research guide looks at where the science is heading.

Our guide to anti-inflammatory foods covers the eating patterns linked with lower inflammation markers.

Where NAD+ and NMN Fit

NAD+ is a molecule every cell uses to make energy, and how it changes with age is an active area of research; our guides to why NAD+ matters for longevity and the NAD+ ingredient page explain the background, and the NMN ingredient page covers its precursor. Some people include NAD+ or NMN in a healthy aging routine. We do not claim that any supplement, including ours, lowers biological age; the foundations above come first. The cellular energy and mitochondrial health hub has more on the topic.

When to Talk to a Doctor

Safety note: a biological age result is not a diagnosis. If a test or calculator gives you an older age than expected, or if you have symptoms such as unexplained weight change, ongoing tiredness or shortness of breath, talk to a doctor, who can check blood pressure, blood sugar, cholesterol and other standard measures. Do not start a strict calorie restriction diet without medical advice, especially if you are pregnant, underweight or have a medical condition.

Frequently Asked Questions

What is biological age?

It is an estimate of how old your body seems based on markers such as DNA methylation or blood measures, as opposed to your age in years.

What is the difference between biological age and chronological age?

Chronological age is the years since you were born. Biological age estimates how far your body has aged, and it can be younger or older than your chronological age.

How is biological age calculated?

Most lab tests use an epigenetic clock, a formula that reads DNA methylation patterns from a blood or saliva sample. Others combine standard blood test results with your age.

Is a biological age calculator accurate?

Free online calculators based on questions are rough estimates. Lab tests are more informative but can still vary by several years between repeat measurements.

Can your biological age be younger than your real age?

Yes. Clocks are built so that results vary around the average for your age, and many people test younger or older than their years.

Can you reverse your biological age?

In a two-year randomized trial, calorie restriction modestly slowed one measure of the pace of aging but did not change other clocks. Habits such as not smoking, staying active and keeping a healthy weight are linked with a younger biological age.

How often should you test your biological age?

There is no set schedule. Because results are noisy, testing every few months is unlikely to show real change; using the same test once a year or less and watching the trend makes more sense.

Does NAD+ lower biological age?

We do not make that claim. NAD+ and aging is an active research area, and the habits in this guide are the best supported starting point.