You have two ages. Your chronological age is how many years you have been alive. Your biological age is how old your body actually is — based on the state of your cells, organs, metabolism, and immune system.
These two numbers can diverge dramatically.1 A 50-year-old marathon runner who sleeps well, eats clean, and has excellent blood work might have the biology of a 38-year-old. A sedentary, stressed, metabolically unhealthy 35-year-old might have the biology of someone a decade older.
Biological age is not a fixed number. It is a trajectory — and it is modifiable.2 That is what makes it the single most important metric in longevity medicine.
Why biological age matters
Chronological age alone is an imperfect predictor of health outcomes. Two 55-year-olds can have radically different risks of heart disease, cancer, cognitive decline, and death.3 Biological age captures what chronological age cannot: the cumulative effect of your genetics, environment, behaviors, and exposures on your actual physiology.4
Research shows that biological age predicts these outcomes even after accounting for chronological age:
- All-cause mortality — how likely you are to die from any cause in a given period3
- Cardiovascular disease risk5
- Cancer incidence5
- Cognitive decline and dementia16
- Physical frailty and disability6
If you could only track one meta-metric for your health, biological age would be it.
How biological age is measured
1. PhenoAge (biomarker-based)
Developed by Morgan Levine and colleagues at UCLA and published in Aging in 2018, PhenoAge uses 9 routine blood biomarkers plus chronological age to estimate biological age:6
- Albumin
- Creatinine
- Glucose (fasting)
- C-reactive protein (CRP)
- Lymphocyte percentage
- Mean cell volume (MCV)
- Red cell distribution width (RDW)
- Alkaline phosphatase
- White blood cell count
These markers span liver function, kidney function, metabolic health, inflammation, and immune status. PhenoAge was built on mortality data from NHANES III (a large US population study) and validated in an independent sample from the later NHANES IV survey,63 and it has since been studied in other cohorts, including the US Health and Retirement Study.4
Advantages: Uses standard blood tests that any lab can run. Cheap. Repeatable. Actionable — you can track changes over time.
Limitations: Does not capture epigenetic changes. Less granular than DNA methylation clocks.
This is the algorithm Merios uses to compute your biological age from any standard blood panel.
2. Epigenetic clocks (DNA methylation)
Epigenetic clocks analyze DNA methylation patterns at specific sites across your genome. As you age, certain CpG sites gain or lose methyl groups in predictable patterns. By measuring hundreds to thousands of these sites, algorithms can estimate a person's age with remarkable accuracy.7
Major epigenetic clocks:
- Horvath clock (2013) — the original multi-tissue clock, built on 353 CpG sites8
- Hannum clock (2013) — blood-based, 71 CpG sites9
- GrimAge (2019) — incorporates DNA methylation proxies for smoking history and plasma proteins; one of the strongest epigenetic predictors of mortality510
- DunedinPACE (2022) — measures the pace of aging (how fast you are aging right now) rather than cumulative biological age11
Advantages: Widely used in aging research.12 Captures deep biological aging signals.
Limitations: Requires a specialized DNA methylation assay. Reliability varies: on six widely used clocks, repeat measurements of the same sample differed by up to 9 years, although newer versions built to fix this agree far more closely.12 Not yet routine clinical practice.13
Companies like TruDiagnostic (TruAge) and Elysium Health (Index) offer consumer epigenetic age testing.
3. Telomere length
Telomeres are protective caps on the ends of your chromosomes that shorten with each cell division. Shorter telomeres are associated with aging, disease, and mortality.14
Advantages: Intuitive concept. Available through consumer testing.
Limitations: High variability between tests. A single measurement is unreliable.15 Telomere length does not change fast enough to track short-term interventions.16 Less predictive of mortality than epigenetic clocks or blood-biomarker scores.10
Telomere testing is interesting but not recommended as your primary aging metric.
What accelerates biological aging
Research has linked several factors to accelerated aging:
- Chronic inflammation — elevated CRP, IL-6, TNF-alpha. Inflammation is the through-line in nearly every age-related disease.17
- Insulin resistance — markers of metabolic syndrome are linked to faster epigenetic aging.18
- Poor sleep — insomnia symptoms are linked to faster epigenetic aging and to signs of immune-cell aging.19
- Chronic psychological stress — linked to shorter telomeres and faster epigenetic aging, possibly through cortisol signaling.2021
- Sedentary lifestyle — lower physical activity is linked to faster epigenetic aging.22
- Smoking — linked to older epigenetic age, most strongly on the GrimAge clock, which includes a DNA methylation signature of smoking.523
- Excess alcohol — alcohol dependence has been linked to faster epigenetic aging in blood and liver.24
- Ultra-processed diet — associated with accelerated biological aging in a large Italian cohort.25
- Environmental toxins — air pollution in particular is consistently associated with older DNA methylation age.26
What slows (or reverses) biological aging
The encouraging reality: biological age is plastic. Some interventions work, although the effects measured in trials so far are modest.2
Exercise
One of the best-supported habits for healthy aging. Higher physical activity is associated with slower epigenetic aging.22 US guidelines recommend 150-300 minutes per week of moderate-intensity activity plus muscle-strengthening work on 2 or more days.27
Diet quality
Mediterranean and whole-food diets have been associated with slower epigenetic aging in several studies.1828 The common thread is high intake of polyphenols, fiber, omega-3 fatty acids, and micronutrients alongside low intake of refined sugar and ultra-processed food.
Sleep
7-9 hours of quality sleep per night.29 In a large study of older women, insomnia symptoms were linked to accelerated epigenetic aging, while self-reported short sleep was not.19
Stress management
Chronic stress is linked to faster epigenetic aging.21 Meditation, breathwork, time in nature, and strong social connections are common ways to manage it, and long-term meditation practice has been linked to slower epigenetic aging in a small study.30
Specific interventions with clinical evidence
- Caloric restriction / time-restricted eating — in the CALERIE trial, two years of calorie restriction modestly slowed the pace of biological aging measured by DunedinPACE, but did not significantly change PhenoAge or GrimAge.2
- Metformin — the proposed TAME trial is designed to test metformin as an anti-aging drug.31 Observational data are promising.32
- Rapamycin — mTOR inhibition with rapamycin extends lifespan in diverse model organisms, and human trials have begun.33
- NAD+ precursors (NMN, NR) — mixed clinical evidence but strong mechanistic rationale.34
- The TRIIM trial — in this small trial of 10 men, a combination of growth hormone, DHEA, and metformin left epigenetic age about 1.5 years below baseline after 12 months (about 2.5 years younger than expected without treatment).35
If you are weighing up which product to buy for this — an epigenetic test, a wearable that estimates an age, or a blood panel that includes one — the prices run from nothing to 849 dollars and the scores are considerably less stable than their marketing suggests. The best longevity apps in 2026 compares them on exactly that: whether the number holds up.
How Merios helps
Upload a standard blood panel to Merios and we compute your PhenoAge biological age automatically — no special test required. Track it over time alongside your Apple Watch data (HRV, resting heart rate, VO2 max, sleep) and see how lifestyle changes move the needle. Your biological age becomes a living, trackable metric, not a one-time curiosity.
Calculate your biological age with Merios →
This article is for informational purposes only. Biological age measurements are tools for health optimization, not diagnostic tests. Consult a healthcare provider for medical decisions.