What biological age is and how it is calculated
Biological age estimates how your body is working, not how long you have left. What PhenoAge measures, which nine blood markers it uses, and how to read the result without over-reading it.
Key points
- Biological age estimates how your body is working compared with what is expected at your age, not how long you will live.
- The most widely used formula, PhenoAge, combines nine routine blood markers with your chronological age.
- It was built to estimate risk across large populations, not to diagnose an individual.
- A single calculation says little: what matters is how the number moves over time.
You are 44 and an app tells you your biological age is 45. Or 39. What exactly does that number mean, where does it come from and what should you do with it?
The short answer: it is an estimate of how your body is working compared with what is expected at each age. It is not a forecast and not a countdown.
Where the idea comes from
Two people of the same age can have bodies that work very differently. Chronological age is measured with a calendar and moves at the same speed for everyone; what differs between people is the state of their systems: metabolism, inflammation, kidney, liver, blood, immune defence.
The idea behind biological age is to summarise that state in a number expressed in a unit we already understand: years.
How PhenoAge is calculated
PhenoAge, also called Phenotypic Age, is the most widely used formula because it only needs common blood tests. It was published in 2018 by Liu, Kuo, Horvath, Crimmins, Ferrucci and Levine in PLoS Medicine, and later validated in more than 11,000 adults from the US NHANES survey.
It combines your chronological age with nine markers:
| Marker | What it mainly reflects |
|---|---|
| Albumin | Nutritional status and liver |
| Creatinine | Kidney function and muscle mass |
| Glucose | Metabolism |
| C-reactive protein (CRP) | Inflammation |
| Lymphocyte percentage | Immune system |
| Mean corpuscular volume (MCV) | Red blood cells |
| Red cell distribution width (RDW) | Variability in red blood cell size |
| Alkaline phosphatase | Liver and bone metabolism |
| White blood cell count | Immune response |
None of them is exotic: all nine come from a complete blood count, a basic chemistry panel and a CRP. In other words, from tests you have probably already had.
What it means if your biological age is higher or lower
If your biological age comes out above your chronological age, the correct reading is: at the time of those tests, the combination of those nine markers looked more like that of older people. It does not say why, or which organ, or what to do.
If it comes out below, that is not a certificate of health either. You can have a low biological age and a condition these nine markers do not look at.
Biological age is not a verdict about you. It is a summary of nine numbers at one moment in time, expressed in a unit you can understand without being a doctor.
What biological age is NOT
- It is not life expectancy. The formula was built on population risk, and the risk of a group does not translate into the fate of one person.
- It is not a diagnosis. No summary number can identify a disease, and that is not what it exists for.
- It is not comparable across methods. There are several biological age formulas, and epigenetic clocks measure something different. Two apps can give you different figures without either being wrong.
- It is not stable day to day. A recent infection moves CRP and white blood cells, and the result with them.
How to use it well
- Look at the trend, not the single value. One calculation is a snapshot. Three or four across a couple of years is a direction.
- Repeat under similar conditions. Same laboratory when possible, fasting, without an active infection and without hard training the day before.
- Do not chase the number. The goal is not to push the figure down, but to understand which marker is moving it and why.
- Take it to your appointment as a question. If something changed noticeably, that is what is worth discussing with your doctor.
Frequently asked questions
Which tests do I need to calculate my biological age?
PhenoAge needs nine blood markers: albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean corpuscular volume, red cell distribution width, alkaline phosphatase and white blood cell count. All of them come from a complete blood count, a basic chemistry panel and a CRP test.
Is it bad if my biological age is higher than my real age?
It is neither a diagnosis nor a sentence. It means that, at the time of those tests, that set of markers resembled that of older people. It may come from something temporary, such as a recent infection raising inflammation, or from a sustained pattern worth reviewing with a health professional.
Why do two apps give me different biological ages?
Because there are several formulas and they do not measure the same thing. PhenoAge uses nine blood markers; epigenetic clocks analyse DNA methylation; other methods include blood pressure or functional tests. Comparing results only makes sense within the same method.
Sources
- Liu Z, Kuo P-L, Horvath S, Crimmins E, Ferrucci L, Levine M. A new aging measure captures morbidity and mortality risk across diverse subpopulations from NHANES IV. PLoS Medicine, 2018
- Corrección de 2019 con la ecuación completa de Phenotypic Age. PLoS Medicine 16(2): e1002760
- Registro del estudio original en PubMed (PMID 30596641)
Keep reading
Why the laboratory reference range is not your personal norm
The lab range describes a population, not you. What those numbers in brackets actually mean and how to read them alongside your own history.
How to read your lab results without panicking
A flagged value is not a diagnosis. What moves your results when nothing is wrong, why something almost always comes back out of range, and how to read your report in five steps.