Biological age
The number that actually moves, and the different ways to measure it.
Your chronological age is the number of birthdays you've had. Your biological age is an estimate of how old your body behaves: how well your blood vessels, metabolism, muscles, immune system and cells are working, compared with the average person your age.
Two people born on the same day can be biologically decades apart. Biological age predicts disease and death better than chronological age does, and unlike your birthday, it responds to how you live.
How biological age is measured
There's no single true number. Different "clocks" measure different things.
| Type | What it uses | Cost | Best for |
|---|---|---|---|
| Blood-marker clocks | Routine blood tests | €30–150 | A cheap, repeatable starting point |
| Epigenetic clocks | Chemical marks on your DNA | €200–500 | Optional: trend only, same lab each time |
| Functional age | Fitness and strength tests | Free | A no-lab estimate, today |
PhenoAge
Developed by Morgan Levine and colleagues in 2018, PhenoAge combines your age with nine standard blood markers: albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red-cell distribution width, alkaline phosphatase and white blood cell count. It predicts mortality considerably better than age alone, and every input is on a normal blood panel.
The Bortz clock
Published by Jordan Bortz and colleagues in 2023, this clock was trained on around 300,000 people in UK Biobank using a wider set of routine blood markers, and it outperformed PhenoAge at predicting mortality. The single most informative marker was cystatin C, a measure of kidney function that most standard panels leave out. Red-cell distribution width, a routine marker, came close behind.
Epigenetic clocks
These read methylation: chemical tags on your DNA that shift with age and with lifestyle. Horvath's clock (2013) was the first. GrimAge predicts lifespan well. DunedinPACE estimates your pace of ageing, in biological years per calendar year, and is the most sensitive to change.
Functional age
VO2 max, grip strength, the sitting-rising test, balance and walking speed each predict mortality on their own, and you can do most of them at home. The Testing & tracking chapter shows how.
Treat it like your weight
A single result is noise. A clock can move a couple of years after a bad week of sleep or a cold. What matters is the trend: the same test, the same lab, the same conditions, six to twelve months apart.
And measure last, not first. If your waist, blood pressure, sleep and step count are off, you already know which way your biological age points. Fix those first. Then test, and enjoy watching the number come down.
Key numbers
9: blood markers in PhenoAge, all on a standard panel.
~300,000: people whose data built the Bortz clock.
6–12 months: the right gap between biological age tests.
This week
- Write down your chronological age and the age you feel, physically. Keep it; you'll revisit it at the end of the course.
- Find your last blood test results. You may already have most of the PhenoAge markers.
- Try the one-leg stand: eyes open, hands on hips, 10 seconds on each leg. Note whether you could.
In the journal
Biological age is an Optimized box in the journal. You don't start there. You start with the cheapest, most obvious red box and work up.
Sources: Levine et al., Aging 2018 · Bortz et al., Communications Biology 2023 · Horvath, Genome Biology 2013 · Belsky et al., eLife 2022 · Araujo et al., British Journal of Sports Medicine 2022