Biological age tests and aging clocks: what a lower number means

Aging clocks, epigenetic age and telomere length are surrogate markers. What a younger result can and cannot tell you, with examples from the radar.

Published 14 September 2026. Built from the radar entries linked below; grades and verdicts update with them.

The short answer

A biological age test estimates how old your body looks from molecules in blood or cells, such as DNA methylation patterns or proteins. A younger result after a treatment is interesting. But none of the clocks used in the studies on this radar has been validated as a stand-in for lifespan or disease, so a lower number is a surrogate result, not proof of a longer or healthier life.

What a surrogate marker is

A surrogate marker is a measurement expected to track an outcome that matters. LDL cholesterol is an established one: the CTT meta-analysis of about 170,000 patients found roughly 10% lower all-cause mortality for every 1 mmol/L reduction in LDL. Aging clocks have not reached that point. Nobody has yet shown that moving a clock changes how long people live.

That is why the radar grades randomised data on markers as B at most, and why a verdict says "no outcome data" when that is the case.

Results on the radar that rest on clocks and markers

Each of these drew attention for a younger biological age or a better marker. Each is graded on what the marker can support.

RentosertibGrade C, post-hoc biomarker. Phase 3, for lung fibrosis.Six aging clocks moved in 42 lung-fibrosis patients over 12 weeks, a design that cannot separate slowed aging from a treated lung, and no healthy-volunteer or outcome data exist.
Therapeutic plasma exchange for agingGrade C, small trial, surrogate. Early clinical.A small trial lowered biological-age scores by about 1.3 years, a surrogate marker, and there is no outcome data.
Lowering epigenetic ageGrade C, associations, surrogate marker. Human biomarker data.A review of interventions associated with lower epigenetic age, a surrogate marker, with no outcome data showing that the stack extends human life.
Hydrogen waterGrade D, single pilot. Early clinical, one pilot.One unreplicated 40-person pilot found longer telomeres, a surrogate marker, there is no outcome data, and most claims come from sellers.

Five questions to ask about any aging clock result

Who was measured? The rentosertib clock result comes from 42 patients with a lung-scarring disease, not from healthy people.
For how long? Twelve weeks says little about a process that takes decades.
Do independent clocks agree? Six clocks from different groups pointing the same way is more convincing than one.
Could something else move the clock? Treating a disease can make blood look younger without slowing aging.
Has anyone measured an outcome? Without disease, function or survival data, the result stays a surrogate.

Where epigenetic age fits

A 2026 review catalogued interventions associated with lower epigenetic age, including exercise, plant-rich diets, semaglutide and caloric restriction. In the CALERIE trial, caloric restriction slowed the measured pace of aging by about 2 to 3% over two years. These are associations and surrogate results, which is why lowering epigenetic age is graded C.

Therapeutic plasma exchange lowered biological-age scores by about 1.3 years in a small Buck Institute trial, and by about 2.6 years when combined with IVIG. Durability and real-world benefit have not been established.

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Not medical advice. Discuss any change with a clinician who knows your history.