Biological Age Explained: What PhenoAge Actually Measures

Your birthday age is not your real age

You and your neighbour might both be 42. But biologically, one of you could be 35 and the other 50. The difference is not visible in the mirror — it is written in your blood.

Biological age — specifically PhenoAge — is a composite measure calculated from nine blood biomarkers and your chronological age. Developed by Morgan Levine at Yale in 2018, it predicts all-cause mortality, cancer risk, and healthspan better than any single blood test.

The nine biomarkers that determine your biological age

PhenoAge uses these markers, each reflecting a different dimension of ageing:

• Albumin — liver function and nutritional status. Higher is younger.
• Creatinine — kidney function. Lower is generally better.
• Glucose (fasting) — metabolic health. Lower is better.
• C-reactive protein (CRP) — systemic inflammation. Lower is better.
• Lymphocyte percentage — immune function. Higher is younger.
• Mean cell volume (MCV) — red blood cell health. Middle of range is optimal.
• Red cell distribution width (RDW) — the strongest single predictor of mortality. Lower is better.
• Alkaline phosphatase (ALP) — liver and bone health. Lower-middle is optimal.
• White blood cell count (WBC) — immune activation. Lower-normal is better.

How the calculation works

The algorithm feeds these nine values plus your chronological age into a Gompertz mortality model — the same mathematics actuaries use to price life insurance. The output is your PhenoAge: the age at which someone with your biomarker profile would typically be found in the population.

If your PhenoAge is 35 and you are chronologically 42, your body is ageing 7 years slower than expected. If it is 48, you are ageing faster.

What the data says

In UK Biobank (502,536 adults), PhenoAge acceleration has a standard deviation of about 9 years in men and 8.8 years in women. That means:

• Top 5% of the population are biologically 14+ years younger than their calendar age
• Bottom 5% are 14+ years older
• Each 1-year increase in PhenoAge acceleration is associated with a 9% increase in all-cause mortality

What you can do about it

The good news: every PhenoAge input marker is modifiable through lifestyle. Mediterranean diet, regular exercise (150+ minutes per week), adequate sleep (7-8 hours), stress management, and not smoking have all been shown to reduce PhenoAge by 1-3 years in intervention studies.

The first step is measuring it. You cannot improve what you do not track.