Biological age vs chronological age

Chronological age is the number of years since you were born: the figure you put on forms, celebrate with cake and use for milestones like voting or retirement. It moves at the same pace for everyone, one year per year, whether you run marathons or mainly run the remote control.

That steady count is still arguably the strongest single risk factor for age-related disease and death.^[2] What it cannot show is how differently two people of the same age are aging. In principle, one 50-year-old could have the heart and blood vessels of a 35-year-old and another those of a 65-year-old, and the calendar would rate them both 50. Biological age is the attempt to measure that difference.

Biological age is an estimate of how far your body has aged. Tests can read it from DNA methylation (small chemical tags on your DNA whose pattern shifts as you age), from routine blood values or from physical tests. Each method captures a different part of aging. Since there is no single true biological age to check them against, two tests can give you two different numbers.

Whatever the method, the estimate reflects wear across your body's systems, shaped by genes, lifestyle, environment and any disease you carry. Unlike your birthday, it differs between people of the same age and can shift within one person over time.

Those estimates also come in generations, and the difference matters when you compare reports. First-generation clocks such as the original Horvath clock.^[1] were trained to match calendar age, using it as a stand-in for biological age^[2] Because they learned the calendar, they predict death risk less well. Newer clocks such as PhenoAge,^[2] GrimAge.^[3] and DunedinPACE^[4] were trained on signs of health, decline or survival instead. They add information about death and disease risk on top of calendar age^{[2, 3, 4]} That holds for groups, though, and for one person a result is a risk hint rather than a diagnosis.

One of those newer names means two things. The PhenoAge blood score (Levine's phenotypic age) combines nine routine blood values with your age.^[2] DNA-methylation PhenoAge, by contrast, is a clock that reads your DNA and was built to predict that blood score.^[2] This guide always says which one it means, and when a report gives you a PhenoAge, it pays to check the same thing, blood draw or DNA test, before you compare it with anything else.

Biological age can be estimated in five main ways, and epigenetic clocks, which read the chemical methylation tags on your DNA, are the most studied. Blood scores, physical tests, telomere length and AI photo analysis each capture a different part of aging. The table shows what each method costs and what its result can tell you.

Prices as of October 2026; the self-pay and dynamometer ranges are our estimates.

Within the epigenetic clocks, the main difference is what the result reports. The original Horvath clock gives an age and works on many tissue types, not just blood^[1] GrimAge also gives an age in years, but one built to predict time to death, coronary heart disease and cancer.^[3] DunedinPACE reports a speed instead: how fast you are aging right now rather than how old you are.^[4]

All three need a DNA sample, and routine blood values offer a cheaper route. Nine of them, including albumin, creatinine, glucose, CRP and blood-count measures, can be combined with your age into one score.^[2] That score is Levine's phenotypic age, also called the PhenoAge blood score.

Physical tests, AI tools and telomeres complete the five. Grip strength, walking speed, balance and cognitive tests show how well your body is holding up. Newer AI tools estimate age from face photos, voice samples or retinal scans. Telomeres, the protective caps on the ends of your chromosomes, shorten in most body cells as those cells keep dividing. Their length tracks age only loosely, though, and is rarely used on its own today.

How far can you trust one result?

In its original test data, the Horvath clock's estimate differed from calendar age by a median of 3.6 years.^[1] That shows how well it tracks calendar age, not how accurate a biological-age result is. A more practical limit is repeatability, meaning whether the same sample gives the same answer twice. Technical noise alone shifted six well-known clocks by up to 9 years between repeat runs, while improved versions mostly agreed within 1.5 years.^[15] Across two samples taken a short time apart, most clocks were only low to moderately stable.^[16]

With wobbles like these, the clocks are research tools, not a clinically validated test like a cholesterol reading. Different clocks can also give different numbers for the same sample. A single result is therefore a rough reading, and a change between two results means more when the same test and the same lab produced both.

The cheapest solid way to estimate your biological age is the free PhenoAge calculator, which needs nine routine blood values and your age.^[2] Home tests such as the sit-rise test and grip strength cost nothing and give risk signals rather than an age. An epigenetic lab test, by contrast, costs several hundred dollars.

The PhenoAge blood score

The calculator asks for your age plus these nine blood values.^[2]:

• Albumin: a liver protein.
• Creatinine: a kidney value.
• Fasting glucose: your blood sugar after not eating.
• CRP: an inflammation marker.
• Lymphocyte %: the share of one type of white blood cell.
• MCV: the average size of your red blood cells.
• RDW: how much the size of your red blood cells varies.
• Alkaline phosphatase: a liver and bone enzyme.
• White blood cell count: how many white blood cells you have.

One of them may already be covered by your health insurance. In Germany, statutory health insurance pays for a check-up every three years from age 35 that includes fasting glucose and a cholesterol panel^[21] Between 18 and 35 there is one check-up, with blood tests only if you have risk factors (as of October 2026).^[21]

Since cholesterol is not one of the nine, fasting glucose is the only PhenoAge value in that package. The other eight need a medical reason, or you pay for them yourself as IGeL (self-pay services). That costs roughly €30 to €80, depending on the lab (our estimate, as of October 2026; ask the practice for a quote).

Once you have the results, match the units before you type. German lab reports often show glucose and creatinine in mg/dL and CRP in mg/L, which is not what the original formula expects.^[2] The table shows the unit the formula needs and how to get there:

Formula units from Levine et al..^[2]; the conversions are simple arithmetic.

Ask for high-sensitivity CRP, because a result shown only as "below 5" cannot be entered.

Home tests

If you have heart disease, chest pain, dizziness, or joint or balance problems, talk to your doctor before the walking, chair and floor tests. Each of the following gives a signal about one part of your fitness, not a biological age:

• Sit-rise test: sit down on the floor and stand back up, ideally without using your hands. You get up to 5 points for sitting and 5 for rising, minus one for each hand, knee or other support, and minus another half point each time you lose your balance a little^[7] Scores of 8 to 10 fell in the lowest-mortality group, while scores of 3 or below went with roughly 5 to 6 times the risk of death over about 6 years.^[7] That figure comes from a study that looked back at data already collected from 2,002 adults aged 51 to 80, and it is a relative risk with a wide range.^[7] Skip the test or keep a chair nearby if your knees, hips or balance are shaky.
• Grip strength: below 27 kg for men or 16 kg for women counts as low muscle strength in the European sarcopenia consensus.^[8] That is a sign of probable sarcopenia (age-related muscle loss), which needs a muscle-mass check to confirm.^[8] The cutoffs are not adjusted for age and do not give you a biological age. A budget home dynamometer (a hand-grip meter) costs roughly €20 to €40 (our estimate, as of October 2026), while medical-grade Jamar-type devices cost more. Our grip strength test then compares your result by age and sex.
• 30-second sit-to-stand: use a sturdy chair with a straight back and no armrests that cannot slide away, its seat about 43 cm (17 in) high. Cross your arms over your chest and count how often you come to a full stand and sit back down in 30 seconds. If you need your arms to get up, stop, because the norms only apply to the standard test.^[23] For men, fewer than 14 stands at 60 to 64 or fewer than 12 at 65 to 69 is below average; for women, the cutoffs are 12 at 60 to 64 and 11 at 65 to 69, per the Rikli and Jones norms.^{[11, 23]}
• Rockport walk test: estimates VO2 max (your maximum oxygen uptake, the standard measure of aerobic fitness) from how fast you walk 1 mile (1.6 km) and your heart rate at the end.
• Fitness age: our fitness age calculator estimates VO2 max from your age, sex, resting heart rate, body size and typical training week, then expresses it as a fitness age.

For a quick number without any equipment, How old do I look? estimates how old your face looks in about a minute. Our lifestyle age test turns your habits into an estimate in a few minutes, and both are rough guides for curiosity rather than measurements.

What a lab test costs

An epigenetic clock test such as TruDiagnostic's TruAge Test (reports include OMICmAge, organ-system ages and DunedinPACE) costs $499 on the US shop as of October 2026.^[22] Check at checkout whether it ships to Germany. Import VAT and possible carrier fees come on top (our estimate: roughly €60 to €150), and your sample and DNA data go to a US lab.

No statutory insurer in Germany, Austria or Switzerland pays for such a test. Our guide on what your insurance does and does not pay for has the details, and our epigenetic test guide compares prices and providers. Given what a single reading can and cannot tell you, we would start with the free blood score before spending that much.

A biological-age result carries real information, and the evidence comes from large groups of people followed over years.^{[2, 3]} In them, people whose clocks ran ahead died earlier and had more coronary heart disease, cancer and Alzheimer's disease, even after allowing for calendar age.^{[2, 3]} That extra risk information is what the newer clocks add to your birthday.^{[2, 3, 4]} It holds for groups, though, and for one person a result is a risk hint, not a diagnosis.

Within those groups, heart health shows how far apart people of the same age can sit. A team analysing US national health survey data scored adults on Life's Essential 8, the American Heart Association's checklist of eight measures.^[9] It covers diet, activity, nicotine, sleep, weight, cholesterol, blood sugar and blood pressure. The researchers then compared each person's phenotypic age (the PhenoAge blood score) with the value expected for their age.^[9]

Adults with the best heart health had a phenotypic age about 3 years below the average for their age.^[9] Those with the worst sat about 3 years above it, a gap of roughly 6 years between the two groups.^[9] A conference abstract using newer survey years found a similar gap.^[10] Both were observational (from watching people, not from experiments), so they show a pattern, not that better heart health caused the younger score.

Whether changing a habit actually moves the score is what intervention trials test, and clocks are increasingly used as their yardstick.^{[14, 16]} A change on one clock can be noise, though, so researchers now look for agreement across several reliable clocks.^{[14, 16]} In a comparison of 18 clocks, the less reliable ones gave variable or misleading results when used to judge interventions.^[16] So a younger reading after a new habit is a reason to keep measuring the same way, not proof that you have aged less.

You can probably lower your biological age a little, but the evidence is thinner than the headlines suggest. Healthy habits go with younger clock results in observational data,^[2] and trials have shifted some clocks slightly.^{[12, 17]} The one pilot that reported a 2-year drop did not hold up when its data were re-analysed.^[14] No study has yet shown that lowering a clock score makes people live longer.^[12]

What the research shows

The everyday habits that slow aging do show up in the clock data, at least as associations. In large groups of people followed over years, more exercise and higher blood markers of fruit and vegetable intake went with a lower DNA-methylation PhenoAge.^[2] Smoking, a high BMI and a high waist-to-hip ratio went the other way, and GrimAge even contains a methylation estimate of how much someone has smoked.^[3] These links come from watching people, so they cannot show that changing a habit moves your number. In our view, the habits themselves (more exercise, more fruit and vegetables, no smoking) are still the part worth acting on, whatever the next reading says.

Whether changing a habit moves the number is what trials test, and so far they have found small shifts. In the CALERIE trial, adults without obesity aimed to eat 25% fewer calories for 2 years.^[12] That slowed DunedinPACE a little but brought no clear change in the DNA-methylation versions of PhenoAge or GrimAge.^[12]

A European trial in generally healthy adults aged 70 and over, DO-HEALTH, also found only small effects. Omega-3 (1 g a day) slowed several clocks, by about 3 to 4 months over 3 years.^[17] On one clock, DNA-methylation PhenoAge, the effects of omega-3, vitamin D (2,000 IU a day) and a home exercise programme added up.^[17] Combining all three moved that clock further than any one alone.^[17] Both trials' clock results also come from post hoc analyses, meaning questions the researchers asked after the trial had been designed for other goals.^{[12, 17]}

The 2-year pilot and its re-analysis

The 2-year figure that travels through news coverage comes from one small pilot trial. In it, 43 healthy men aged 50 to 72 followed an 8-week diet, exercise, sleep and relaxation programme or no programme.^[5] On the Horvath clock, the treated men came out about 2 years younger, while the control group's clock age rose slightly over the same weeks.^{[5, 6]}

The gap between the two groups, a little over 3 years, was the trial's clear result.^{[5, 6]} The treated men's own drop was shakier: it only just cleared the usual test for chance after a later correction, having fallen short in the original paper.^{[5, 6]} Fewer than half of the treated men got younger at all.^[6]

Those are not the only caveats, and the others are big. The trial was funded by Metagenics, which makes the two supplements the men took.^[5] Two authors also use the programme commercially and are named in a related patent application.^[5] When other researchers re-analysed the same data, none of 11 other clocks confirmed the drop, and the original result itself no longer held up statistically.^[14] Until a larger trial that includes women repeats it, the pilot is a reason to keep watching rather than a result to plan around.

What the study group did (Fitzgerald 2021)

The treated men in that pilot were healthy men without chronic disease, and they had coaching sessions weekly at first, then at least every other week.^[5] Their programme looked like this,^{[5, 6]}:

• Daily foods: 2 cups of dark leafy greens, 2 cups of cruciferous vegetables, 3 more cups of colourful vegetables, 1 to 2 beets, ¼ cup each of pumpkin and sunflower seeds, 6 oz (about 170 g) of animal protein foods such as meat or fish and 2 servings of low-glycaemic fruit.
• "Methylation adaptogens": at least 1 serving a day, such as ½ cup of berries, ½ tsp of rosemary or turmeric, 2 garlic cloves, 2 cups of green tea or 3 cups of oolong tea.
• Weekly foods: 5 to 10 eggs and 3 servings of liver (3 oz, about 85 g each).
• Eating window: no food between 7 pm and 7 am.
• Cut out: added sugar, dairy, grains and legumes.
• Exercise: 30 minutes on at least 5 days a week at 60 to 80% of maximum perceived exertion (based on how hard it feels, not on heart rate).
• Sleep: at least 7 hours.
• Breathing exercises: twice a day.
• Supplements: two Metagenics products, PhytoGanix greens powder, 2 servings a day, and UltraFlora Intensive Care (Lactobacillus plantarum 299v), 2 capsules a day.

Before you try this: Liver is very rich in vitamin A: an 85 g serving of beef liver has about 8,000 µg^[19] so three servings a week average above the adult upper limit of 3,000 µg a day.^[18] EFSA advises keeping liver to about once a month, and avoiding liver completely if you are pregnant or planning a pregnancy.^[18] If you take warfarin or another vitamin K antagonist (in Germany often phenprocoumon, Marcumar), talk to your doctor before sharply increasing leafy greens and cabbage, because they change how the drug works.^[20] Check with your doctor first if you take diabetes medication or have a history of eating disorders, since the plan cuts carbohydrates and adds a 12-hour overnight fast.

The result behind this programme did not hold up in the re-analysis, so treat the list as a description of the study, not a recipe.

How often to retest

A single lab reading can wobble by a year or more,^{[15, 16]} so a lower number on its own does not prove your health improved. Between lab tests, grip strength^[8] or the sit-rise test^[7] can be repeated monthly at no cost, which gives you something to track while you wait. For the lab test itself, we suggest waiting 6 to 12 months between readings, a practical rule of thumb rather than a validated one. Use the same test and the same lab each time.