What is biological age?
Chronological age counts birthdays. Biological age attempts to answer a harder question: how old does your body appear to be functioning right now?
The basic idea
Two people born in the same year can differ by a decade or more in arterial stiffness, insulin signalling, cardiorespiratory fitness and kidney reserve. Chronological age cannot express that. Biological age is an attempt to.
Unlike your birthday, it can move in either direction. That is the entire reason the concept is useful — it implies something actionable, where chronological age implies only the passage of time.
Two families of method
Epigenetic clocks measure DNA methylation — chemical marks on DNA that change in patterned ways with age. They are measured from blood or a cheek swab and produce a single number. They are research-grade, interesting, and expensive, and they do not tell you which system is drifting or what to do about it.
Biomarker-based methods score measurements you can obtain from ordinary blood work, a wearable or a physical test. They are less exotic and considerably more actionable, because every input is something you can influence and re-measure.
This tool is the second kind. It is not an epigenetic clock and does not claim to be comparable to one.
Why one number is not enough
A single composite age hides the thing that matters most. Your vascular system, metabolism, brain and musculoskeletal system age at different rates, and an average across them can look reassuring while one drifts badly.
Scoring each system separately makes the disagreement visible — and the disagreement is usually the most useful output. A composite of 42 built from seven systems at 38 and one at 61 is a very different situation from eight systems all at 42, and the single number cannot distinguish them.
What the number cannot tell you
It is not a diagnosis, and no biological age method is clinically validated as a diagnostic instrument.
It cannot see what you have not measured. A good score on a thin panel means the panel was thin — which is why a method that quietly fills gaps with assumptions is worse than one that excludes them and says so.
It is also not a prediction about you individually. These models are built on population data, and population-level associations do not determine any one person's outcome.
What makes a method trustworthy
Ask three questions of any biological age tool. Where do its reference ranges come from? What does it do when data is missing? And does it return the same answer twice?
The third is easy to overlook and decides whether you can track change at all. If the method drifts between measurements, the difference you observe is partly you and partly the method, and nothing tells you which.
Score yourself free — no account needed →Frequently asked questions
What is biological age?
Chronological age counts birthdays. Biological age attempts to answer a harder question: how old does your body appear to be functioning right now?
What does this guide cover?
This guide covers: The basic idea; Two families of method; Why one number is not enough; What the number cannot tell you; What makes a method trustworthy.