The PDF arrives on a Tuesday morning, between a calendar invite and a shoe sale. You print it, which you never do, and take it to the kitchen table with a pen and a coffee going cold.

The first line says your body is four years older than your passport. You check that they have the right birthday. Then you start writing in the margins, which is the correct way to read one of these letters.

A single biological-age result reflects where you are in life as much as how you are aging, so the only number worth acting on is a trend measured at comparable moments.

Woman at a kitchen table annotating her biological age test results letter with a pen, coffee beside her

The short answer: Biological age test accuracy is limited for women, because a single result reflects life stage as much as aging. Pregnancy, surgery and severe illness can push epigenetic age up for a while, and older clocks can disagree with themselves by years on the same sample. Compare only results from the same lab, clock and sample type, taken at similar moments.

  • Harvard-led study, Cell Metabolism 2023: epigenetic clocks rose through surgery, severe COVID-19 and pregnancy, then fell after recovery
  • Yale, 2024: biological age dropped by as much as eight years for some women by three months postpartum
  • DO-HEALTH, Nature Aging 2025: omega-3 slowed aging clocks by roughly three to four months over three years in adults 70 and older
What was happening when you spat What the clock does How sure are we Good moment to test?
Pregnancy or the months after birth Runs fast, then largely rewinds after delivery Emerging No
Surgery or a severe illness Jumps, then recovers with you Emerging Not until you are well
Menopause, especially early Reads older, and the shift appears to last Emerging Yes, compared only with the same stage
Nothing at all Older clocks can still disagree with themselves by years Established Yes, with the same lab and clock each time

Line one: "Biological age, 41"

Your margin note here is a question mark, and the history of this number deserves one. The science behind it began as a failed experiment in sexual orientation.

Saliva DNA collection tube and funnel for a biological age test on a laboratory bench beside a pipette

At UCLA, the statistician Steve Horvath volunteered to analyze a colleague's saliva samples from identical twins in which one twin was gay and the other straight. "There was no signal whatsoever for homosexuality," he later told Quanta. Then he looked at the same data for age, and the pattern "blew me away."

The signal lives in DNA methylation, the chemical tags on your genes that shift as the years go by. An algorithm called an epigenetic clock compares your pattern with thousands of others and produces an estimate. Horvath's summary was cheerfully blunt: "You spit in a cup, and we can measure your age." What the letter calls your biological age is a statistical estimate from those tags, and a good one, but an estimate all the same.

Line two, "Method: PhenoAge": how accurate is a biological age test?

Circle the name of the clock. Your friend's report may say GrimAge or DunedinPACE. Those are different instruments trained on different questions, so comparing hers with yours is like comparing a bathroom scale with a tape measure.

New mother holding her sleeping newborn beside a bright window in a quiet room

Biological age test accuracy is also less steady than the typeface suggests. When researchers ran the same samples twice through several popular older clocks, the readings could drift apart by years. Nothing in the donor had changed. The spit was identical, and the machine had a different opinion on Thursday.

Your next test only means something if it comes from the same company, the same clock and the same kind of sample. Newer versions built to correct this drift exist, so ask which one you are getting before you pay.

Line three: "Sample collected: 14 March"

Most people skip the date. Write beside it what was going on that week, because the clock was listening.

In 2023, a Harvard-led team published a paper whose title reads like a margin note of its own: "Biological age is increased by stress and restored upon recovery." Clocks climbed through major surgery, severe COVID-19 and pregnancy. Then they fell again once the stress had passed.

Anyone who has shared a cottage with their in-laws for a long weekend will recognize the team's strangest result, which came from mice. When a young mouse's circulation was surgically joined to an old one, the young mouse aged on the clock. Once the pair were separated, it bounced back.

Pregnancy got its own study at Yale the following year. Kieran O'Donnell's team followed women from early pregnancy until three months after birth. The clocks ran fast, then sharply backwards. "At three months postpartum, we saw a remarkably large decrease in biological age, by as much as eight years for some individuals," he told Yale News. Women who breastfed saw a steeper fall.

A test taken in late pregnancy or the fourth trimester records an event in progress, and it makes a misleading baseline for everything that follows.

Line four, "Recommendations": can supplements lower your biological age?

Some reports end with a shopping list. Before you underline anything, look at the best trial anyone has run on this question.

DO-HEALTH, a large European study, gave adults aged 70 and older vitamin D, algae-based omega-3, a simple home exercise routine, or combinations, for three years. Omega-3 slowed several clocks, and the three together added up. The benefit came to roughly three to four months of slower aging over three years. The volunteers were also decades older than you.

A few months of clock time is well inside the drift from line two, so no single consumer test will show you a supplement working. The real reason to take these is the ordinary one, which is that they are good for you in their own right.

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The last line: "Retest in six months"

Of everything on the page, this line deserves your signature, with conditions added in your own handwriting. Use the same lab, the same clock and the same sample type, in a season of life that resembles this one.

Closed paper folder on a wooden table with a pen on top and a scatter of golden omega-3 softgels

A first result taken during a hard month becomes the ruler for every result after it. You could spend a year and a small fortune chasing a gap your body was already closing on its own. Put the letter in a folder, write the date of the next test on the front, and leave the pen on top.

This article is general information and does not replace advice from your doctor or pharmacist.

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Frequently asked questions

Is a saliva or cheek-swab test as good as a blood test?

They measure different tissues and can disagree. In a 2016 PNAS study, the link between earlier menopause and older epigenetic age appeared in blood but not in cheek cells or saliva. Switching sample type between tests means comparing two different measurements.

Does hormone therapy make you biologically younger?

One analysis hints at it. The same 2016 study found lower epigenetic age in the cheek cells of women who had used menopausal hormone therapy. That is an association in one tissue, and no reason to start or stop hormones without your doctor.

How much vitamin D is too much?

Health Canada sets the recommended intake for women aged 19 to 50 at 600 IU a day and the upper limit at 4,000 IU a day from all sources. DO-HEALTH used 2,000 IU, so count any multivitamin before adding more.

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