From the beginning of time, humankind has searched for the secret to a long life. Now science may have found an answer, in the form of molecular augury. The pattern of chemical chains that attach to the DNA in your cells—on-off switches known as epigenetic markers—can reveal how swiftly you are aging, and perhaps even how much longer you will live. While genetic testing might tell you where you came from, epigenetics promises a glimpse into the future. Now, a handful of companies are offering commercial blood or saliva tests based on the science of epigenetics—a chance to find out how old you truly are.

DNA itself is fixed; the genes you’ve inherited will forever make you more or less prone to certain conditions. But your epigenetic patterns change based on what you eat, how much you sleep or exercise, and what substances you are exposed to—and ultimately influence your risk of developing chronic conditions such as heart disease or diabetes. They do so by altering gene activity, like a complex set of controls that turn genes up or down, on or off.

Epigenetics plays a role throughout life, from embryo development to aging, and some epigenetic changes accumulate as the years pass in a way that literally inscribes your age upon your body. That process is influenced by your environment and the things you do or don’t do. If you adopt a new exercise regimen or are exposed to pollutants, your epigenetic patterns may change for the better or worse.

In 2011, after analyzing hundreds of epigenetic markers, Steve Horvarth developed the first epigenetic “clock,” a new way to calibrate aging. It measures specific epigenetic patterns that are linked to aging and disease, and compares that result against what would normally be expected for someone of your age. You may look or feel younger than expected, but this clock puts a number on it and tells you whether you really are aging more slowly than most other people.

That number, what’s sometimes called your biological age, might be older or younger than the years you count on each birthday. Fifty can be the new 40, or 50 can be more like 60.

Horvath, a biostatistician at the University of California Los Angeles, wasn’t trying to create a scientific oracle. A test of cellular aging, he reasoned, would be a powerful tool for researchers trying to find ways to prolong life and health. He created the test by focusing on methylation and demethylation, processes that grow or shrink those chemical chains and thereby control the action of genes. His clock accurately predicted biological age when tested in groups of hundreds of people, giving researchers an efficient way to look for the longevity benefits of drugs or lifestyle changes.

At first, he had trouble publishing his results. It seemed too good to be true that a simple blood test could accurately predict lifespan, says Horvath. But other studies backed up his findings. He has refined the test over the years, and says his lab’s newest test, dubbed GrimAge for the Grim Reaper, is the most accurate yet. (He developed GrimAge for for research, although some version of it could become publicly available in the future.)

The science of an epigenetic clock is now well-established. (There are other clocks in addition to Horvath’s.) The commercial tests to predict your biological age are based on this science—but these proprietary products aren’t independently evaluated. For example, they might not be equally accurate for all ethnic or racial groups.

Still, they can give you some interesting information. Spit into a tube or prick your finger to take a few drops of blood, and send the samples off along with $299 or $500 depending on the test. If you smoke or if you are obese, your epigenetic markers are likely to be biologically older. If you are fit and have a healthy diet, you may detect the benefits.