Biological Age vs Chronological Age

Two ways to count age

Chronological age is the number of years since birth. Biological age is an estimate of how the body is functioning compared with other people, based on measurements such as blood tests or chemical marks on DNA. The idea is that two 55-year-olds can differ in how "worn" their bodies are. The World Health Organization (WHO) says the biological changes of aging are only loosely associated with a person's age in years. [1] This page is education only. It does not interpret anyone's test results, and it does not recommend testing.

Evidence labels follow how we label evidence in longevity research.

Kinds of biological age estimates

A review of the field described six types of candidate predictors: epigenetic clocks, telomere length, transcriptomic (gene activity), proteomic (proteins), metabolomic (small molecules in blood) and composite biomarker predictors. [2] Another review grouped calculators into those using standard clinical and laboratory markers, molecular markers and epigenetic markers. [3]

What the evidence says

• Promising but not proven. A 2017 review concluded that the epigenetic clock was the most promising new biological age predictor but that its true value as a biomarker of aging requires longitudinal confirmation (follow-up over time). [2] The same review said the search for reliable indicators had gone on for over three decades, largely without success until recently. [2]
• No single universal measure. A 2022 review stated there is no single universal biomarker to estimate overall health status and longevity prospects, and that a consensual approach to the very concept of aging and its assessment is still to be developed. It found that calculators combining many markers were most accurate, but most need further improvement and cannot yet be considered for standard clinical practice. [3]
• No consensus standards. A 2023 framework from the Biomarkers of Aging Consortium said the current lack of standards and consensus on what makes an aging biomarker reliable hinders its further development and validation for clinical applications. [4]

Label: promising but limited as research tools; insufficient evidence that any biological age estimate should guide medical decisions for an individual.

Three questions to ask of any "biological age" result

1. Is the measurement accurate and repeatable? (analytical validity)
2. Does it predict health outcomes in people like you? (clinical validity)
3. Does acting on it improve your health? (clinical utility)

The reviews above say validation of biological age estimates is still incomplete, and the consortium paper lists validation as an area needing future research. [4] Whether a changing number means better health is a separate question: a change in a marker is not the same as a change in disease, function or survival. The testing details are on biomarkers and biological age tests.

What to do with this

Biological age is a research concept, not a diagnosis. If you are curious about your own health as you age, a clinician can discuss established checks and screening. Everyday habits are covered in the Wellness guide to healthy aging. For the broader picture of the biology, read what is aging.