Biological age: why a single number settles nothing

Biological age is an attempt to describe, in one number, how far a body has travelled along ageing. The idea is easy to follow: the calendar is the same for everyone, bodies are not. The trouble starts when that number is read as a verdict or as a prize.

Why the estimates disagree

Biological age is not measured — it is estimated, by a model. There are many models, built on different inputs: some on blood chemistry, some on physical measures, some on epigenetic marks. Each carries its own assumptions and its own training population, so two calculators can legitimately return different numbers from the same lab panel. That is not one of them being broken; they are answering slightly different questions.

What to do about it

Watch the direction, not the absolute value — and only inside a single model. "Three years younger" between two different methods means nothing. The same estimate, computed twice six months apart by the same method, does say something.

And remember the model computes from what you gave it. A missing marker does not make the result more honest; it makes it more confident than it deserves to be.

A longer explanation of how these estimates are built is in Biological age: what it is and how it is measured.