Chronological age is the number on your passport. It goes up by exactly one every year and tells you nothing about how your body is doing. Biological age is different. It is an estimate of how old your body looks, biologically, compared with other bodies.

The idea is simple. Take a large number of people whose chronological ages are known. Measure something in each of them, such as patterns of chemical marks on DNA, or the levels of thousands of proteins in blood. Then train a model to predict age from those measurements. Once trained, the model can be shown a new sample and asked: how old does this look?

If the answer is lower than your chronological age, your biology resembles younger people in the reference group. If it is higher, it resembles older people. The gap between the two numbers is what people usually mean when they talk about aging faster or slower.

A few things are worth being clear about.

First, a biological age is an estimate from a model, and models have error bars. A single reading is a snapshot with some noise in it. The trend across repeated readings is more informative than any one value.

Second, different methods produce different numbers. An estimate built from proteins is not the same as one built from DNA methylation, and neither is a fixed truth. What matters is using the same method consistently so readings can be compared.

Third, biological age is an indicator of wellness, not a diagnosis. It does not tell you that something is wrong, and it does not replace the tests your clinician orders. It gives you a sense of direction.

Why proteins? Because proteins are close to the action. They are the molecules that actually do the work of the body, and their levels shift with sleep, diet, training, illness and recovery. Research groups have reported that protein patterns in blood track age closely, and that they also carry information about individual organs. That second point is the subject of the next article.