Two people, both 45 years old. One runs marathons, sleeps well, has good metabolic health, and low inflammation. The other has been sedentary for a decade, has elevated fasting insulin, chronic low-grade inflammation, and suboptimal nutrient status. They are the same age by calendar. They are not the same age biologically.
Biological age — sometimes called physiological age or phenotypic age — attempts to measure how well your body is actually functioning relative to chronological age. It's not a single number from a single test. It's an inference from a cluster of markers that correlate with the ageing process.
The Hallmarks of Ageing
In 2013, a landmark paper by López-Otín and colleagues identified nine biological hallmarks of ageing: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. A 2023 update added three more.
These hallmarks describe the molecular and cellular mechanisms through which ageing occurs. Many of them produce measurable signals in the blood.
Biomarkers that track biological ageing
No single test perfectly captures biological age, but a panel of markers provides a meaningful picture:
PhenoAge — developed by Dr Morgan Levine at Yale, this is a composite score calculated from nine clinical biomarkers including albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase, and white cell count. It correlates with mortality risk better than chronological age alone.
GlycanAge — based on the glycosylation pattern of IgG antibodies, which changes with inflammation and biological ageing. Glycans can be measured from a blood sample and produce an age estimate that has been validated against health outcomes in longitudinal studies.
Klotho — a protein produced primarily by the kidney whose blood levels decline with age. Higher Klotho is associated with better cognition, cardiovascular health, and kidney function. Measurement from blood is now clinically available.
Telomere length — telomeres are protective caps on chromosomes that shorten with each cell division and with oxidative stress. Short telomeres are associated with accelerated ageing and higher disease risk. Telomere length can be estimated from blood samples, though the methodology varies between tests.
hs-CRP and inflammatory markers — chronic low-grade inflammation is one of the most consistent features of biological ageing ("inflammaging"). CRP, IL-6, and fibrinogen all track systemic inflammatory burden.
IGF-1 and growth factors — IGF-1 declines with age and is involved in cellular repair and metabolism. Its trajectory over annual testing reflects changes in metabolic and hormonal status.
Why annual testing matters for biological age
Biological age estimates from a single test are interesting but limited. The value compounds with repeated measurement.
If you change your training, sleep, nutrition, or supplementation, you want to know whether it worked. A second test one year later, using the same methodology, can show you whether your PhenoAge improved, whether your glycan profile shifted toward a younger pattern, whether your inflammatory markers came down.
The goal isn't to know a number. It's to have a feedback loop — so that the choices you make in the next year are informed by evidence from the last one.
foreverbetter's panel includes PhenoAge, GlycanAge, telomere length, Klotho, and the full inflammatory marker set — not as curiosities, but as the foundation of an annually retested biological age profile.