Chronological age moves at a constant rate. Every year adds another year to our age.
But does our biology age at the same steady pace?
Not necessarily.
Many methods used to estimate biological age rely on biomarkers that change with chronological age. Some models assume that the relationship between a biomarker and age is approximately linear — meaning that the biomarker changes in a relatively predictable way as people get older.
Biological aging may be more complex.
Some biomarkers can show nonlinear changes with age. A biological measurement may change slowly during one period of life and differently during another, rather than following the same pattern throughout adulthood.
People can also age differently from one another.
Two people who are both 60 years old have lived for the same number of years, but their biological profiles may not look the same. Differences in aging-related biomarkers can contribute to different estimates of biological age.
There is another complication: different parts of the body may not age at the same rate.
Aging-related changes can differ between tissues and organ systems. This makes it difficult to assume that one biological-age number necessarily represents aging throughout the entire body in exactly the same way.
Why does this matter when biological age is calculated?
If researchers select only biomarkers that have a simple linear relationship with chronological age, they may overlook potentially useful biomarkers that change in more complex ways.
The mathematical model matters too. A model built around a steady relationship between age and biomarkers may not fully capture biological changes that vary over time or between different parts of the body.
This is one reason why estimating biological age remains challenging. Chronological time is uniform. Biological aging may not be.
