Biological age tests compared: what they measure and what they don't

Two fifty-year-olds can age at very different speeds. Biological age tests try to capture that difference in a single figure. Used well, they give you a starting point and a way to see whether changes in your life move the needle. Used badly, they become an expensive horoscope.

Epigenetic clocks read DNA methylation — chemical marks on the genome that shift in predictable ways as we age. The first-generation clocks, such as Horvath's, estimated chronological age. Newer clocks were trained on health outcomes instead:

• PhenoAge and GrimAge link methylation patterns to disease and mortality risk.
• DunedinPACE estimates the current pace of aging — how many biological years you age per calendar year.

For tracking change over time, pace measures are often more useful than a single age figure. The limits: results vary between labs and algorithms, and small changes between two tests may sit within measurement noise.

Telomeres shorten as cells divide, which made them an early favorite. In practice, telomere length varies widely between people of the same age and predicts individual outcomes poorly. Most researchers now see it as a weaker marker than epigenetic clocks.

Recent research uses blood proteins to estimate the age of individual organs — heart, brain, kidney and others. Early studies suggest that an organ aging faster than the rest predicts disease in that organ. It's promising, but still mostly a research tool.

Some of the strongest predictors of how long and how well you live are not exotic at all: VO2 max, grip strength, walking speed and balance. They are cheap to measure, respond to training, and tell you directly what to work on.

A consumer epigenetic test typically costs a few hundred dollars. As part of a physician-led longevity workup, the price is usually bundled. Functional tests such as VO2 max testing often cost less than a methylation test and arguably tell you more you can act on.

Test once for a baseline, change one or two things that matter, then retest with the same lab and the same clock after six to twelve months. Never compare results across different tests.