How Old Are You Really? Part II. Reading Your Inner Clock
Archaeologists know a secret: our bodies tell time. And your teeth? They might be the best record keepers of all.
Like the rings of a tree, our teeth record the rhythms of our earliest days. Imprinted not in bark, but in precise growth lines laid down daily and etched into enamel and dentin. Using microscopes or advanced imaging, scientists can count these lines to estimate age with astonishing precision. To the trained eye, even a single molar can share stories of growth, stress, starvation, or long-forgotten illness.
I first learned about the fascinating story of teeth as timekeepers when I was an undergraduate anthropology student. Back then, I was trying to make sense of time in a different way. I spent my days working in a genetic anthropology lab, pipetting ancient DNA to help uncover how tuberculosis evolved throughout human history. But that was a past life.
It seems full circle now (and odd that I never connected these experiences until I started writing this), but I’ve traded the antiquated for the living and ancient DNA for real-time biomarkers. Nonetheless, the question remains the same: how do we measure time in the body? Today, that question has new tools and a new sense of urgency.
In Part 1 of this series, I wrote about the difference between biological and chronological age and how two people born the same year might age in radically different ways.
Now in Part 2, we’ll explore how scientists measure biological age, not by counting tree rings or examining molars, but through blood markers and DNA methylation patterns.
Let’s Quickly Recap What We Learned in Part 1.
Chronological age is the number of years you’ve been alive.
Biological age is how “old” your body really is on the inside.
Biological age gives us a better sense of how healthy we are, how fast (or slow) we’re aging, and it can even predict future risk of disease and death more accurately than your birth certificate ever could.
So how do we measure it? Well let’s back up for a second and talk about what makes a good measure of biological age. Why? Because there are a lot of tests out there, and we want to have the tools to decipher between which ones are accurate versus effective marketing.
What Makes a Good Test for Biological Age?
Scientists agree that a good biological age test should:
Reflect one or multiple hallmarks of aging (see Part 1 for a refresher)
Predict risk of disease, disability, and death
Be easy to collect (non-invasive, if possible)
Work across diverse populations
Respond to interventions that can speed up or slow down aging
What is the gold standard test? Well, it doesn’t exist… yet.
There are numerous ways to measure biological aging, but for brevity purposes, I will focus on a few solid contenders. Let’s break them down.
1. Phenotypic Age: Lab Tests Reimagined
Think of this as a remix of your standard blood work. Researchers developed algorithms that take common lab tests, like blood sugar, inflammation markers, and kidney function, and use them to estimate your body’s internal age. One of the best known is called Phenotypic Age (or PhenoAge), which has been shown to predict who is more likely to get sick or die, even better than chronological age.
2. Epigenetic Clocks: DNA That Tells Time
Epigenetics control which parts of your DNA are switched on or off. Scientists have discovered that patterns of DNA methylation, or tiny chemical tags on your genes, change as you age. They predict morbidity and mortality better than chronological age and have become popular to use in research studies because: 1) all you need is some blood, and 2) epigenetics are thought to be reversible, so repeated tests over time could show that someone has slowed down their aging.
The downside to the epigenetic clocks is that they were created from algorithms, so their underlying mechanisms are unknown (meaning, we aren’t sure which hallmarks of aging are driving these clocks). Also, it is quite common that if you use different epigenetic tests, you will likely get different results, since these clocks use different combinations of DNA methylation, which make it hard to know what your real biological age is. Nevertheless, these clocks remain an important tool.
Some of the big names in this space include:
Horvath Clock (the OG epigenetic clock)
DNAm PhenoAge (predicts morbidity and mortality)
GrimAge (predicts time to death… cheery name, important tool)
DunedinPACE (the only epigenetic clock that tells you how fast you are aging, year by year)
3. Cellular Senescence: The Zombie Cell Problem
Did you know that you have zombie cells in your body? They don’t eat brains (thankfully), but they do cause trouble. These so-called senescent cells stop dividing and can linger long after they should have died, spewing inflammatory signals and damaging nearby healthy cells. Scientists call this cellular senescence, but let’s be honest… it sounds like something straight out of The Walking Dead. Except instead of roaming the post-apocalypse, these “undead” cells are hanging out in your joints, your fat tissue, maybe even your brain. They slow regeneration, accelerate aging, and silently stir up chaos. Don’t worry, this isn’t a horror movie… yet.
Unlike other biomarkers, measures of cellular senescence, like p16INK4a, provide a direct readout of a primary aging hallmark (cellular senescence) and can be measured in tissues or in blood with markers. However, these metrics are not widely available yet outside of research labs.
Cool, But I Want to Know My Biological Age.
As promised, here are some options to assess your biological age:
Free & Simple: Ask yourself, do I feel younger or older than my actual age?
Surprisingly, subjective age (how old you feel) is a decent proxy for biological age and brain aging. So, now I am curious, how did you respond?Free With Some Effort (and your data may be used): Remember PhenoAge? You can gather your own lab data by asking your doctor to order you some simple blood tests (CMP, CBC & hs-CRP panels) and plug the results into an online calculator like this one from AgelessRX (or search for others online). A list of the blood test results needed to calculate PhenoAge are provided below.
Blood Test Results (units) Needed to Calculate PhenoAge:
o Albumin (g/dL)
o Creatinine (mg/dL)
o Glucose (mg/dL)
o C-reactive protein (mg/L)
o Lymphocyte percent (%)
o Mean cell volume (fL)
o White blood cell count (cells/mL)
o Red cell distribution width (%)
o Alkaline phosphatase (U/L)
Wearables & Apps (Low Cost): Some health apps use your heart rate variability or sleep data to estimate your biological age. Take these with a grain of salt, many are proprietary algorithms and aren’t scientifically validated in peer-reviewed studies.
Epigenetic Age Tests (High Cost): Companies like TruDiagnostic, Tally Health, Zymo Research, and myDNAge offer at-home kits to estimate your biological age using DNA methylation. These tests typically cost $200–$500.
Here's a quick look at which of the best studied clocks are available through commercial providers:
Disclaimer this post is not sponsored, and I have not personally used any of the tests or worked with the companies listed above. I am just sharing options.
Where We’re Headed: The Future of Aging
Where is all of this going? Here is my take on what is likely coming down the road, and probably sooner rather than later.
Reprogramming Cells
In labs, scientists have already learned how to “reset” the biological age of cells by activating certain genes, essentially making them younger. In mice, this process has reversed age-related blindness and even improved brain and muscle function.
Senolytics: Clearing Out the Cellular Junk
Remember those zombie cells? There’s now a class of drugs called senolytics designed to seek them out and destroy them. In early trials, these drugs have reduced inflammation and improved physical function in older adults. The goal is to clear the clutter and let healthy cells thrive.
Multi-Omics Super Clocks
Future biological age tests won’t just look at your DNA. They will combine data from your genes, proteins, metabolites, immune system, microbiome, alongside functional measures like walking speed and grip strength to create a sort of biological age super clock that paints a more complete picture of aging. Imagine checking your biological age the way you check your blood pressure.
Precision Aging
If I had to make a prediction, I’d say in the not-so-distant future, your doctor (or AI health coach) might say something like: “Based on your biological age, we’re going to fine-tune your exercise, nutrition, sleep, and maybe prescribe a senolytic. Let’s get your aging rate under 1.0.” Think of it like preventative medicine meets a master tailor, adjusting every seam and stitch to fit your biology perfectly.
Final Thoughts
Just as enamel patterns in teeth divulge nature’s timestamp, scientists today are reading new kinds of biological records. From epigenetic clocks to phenotypic algorithms and senescence markers, we now have powerful tools to estimate how well we’re aging, not just how long we’ve been alive. These measures aren’t perfect, but they offer a more dynamic, personalized view of health. They haven’t just changed the game, they’ve completely rewritten the rules on how we think about and will approach aging, prevention, and longevity in the future.
In Part 3, we’ll turn from measurement to action and discuss what you can do to slow or even reverse your biological age.
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For your reading pleasure:
Margolick & Ferrucci, 2015, “Accelerating aging research: how can we measure the rate of biologic aging?” Exp. Gerontol.
Horvath & Raj, 2023, “DNA methylation-based biomarkers and the epigenetic clock theory of ageing”, Nature Reviews Genetics
Horvath, 2013, “DNA methylation age of human tissues and cell types”, Genome. Biology
Levine et al., 2018, “An Epigenetic Biomarker of Aging for Healthspan and Lifespan”, Aging
Lu et al., 2019, “DNA methylation GrimAge strongly predicts lifespan and healthspan.” Aging
Belsky et al., 2022, “DunedinPACE, a DNA methylation biomarker of the pace of aging.” eLife