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There's no scientific test that can determine your age

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Back during the Beijing Olympics in 2008, critics accused members of China's gymnastics team of being below the age limit of 16. In gymnastics, being small and light can be a serious advantage, so there's some incentive to lie about age.

Unfortunately, "age doping" may be one sports scandal that can never be fixed. While it's possible to test athletes for drugs, there's actually no scientific test to determine someone's exact age.

This might seem like a weird statement, at first. After all, we can tell how old trees are by measuring their rings. But there's no comparable test for humans. If a person doesn't have their birth certificate, there's no way to reliably tell his or her age. And it's a problem that comes up quite often. For example, someone might want to know how old a person claiming retirement benefits actually is. Or want to estimate how old a crime victim was. Or figure out whether an immigrant is really a minor or not.

For years, scientists tried everything they could think of — looking at growing bones, white blood cells, and so on — to verify people's ages. Nothing worked. More recently, scientists have developed something called the "epigenetic clock," which can get within plus or minus two or three years. But a more accurate test is still elusive. Here's a rundown.

Scientists have tried (and failed) to find ways to verify ages before

There are several different approaches here:

1) Looking at bones. For children, the rate at which bones grow can sometimes provide a clue to age. For example, one study by a FIFA soccer researcher used MRI to examine the growing bones in the wrists of children. It was able to determine with 99 percent accuracy whether a person was older than 17 (and therefore too old to play in certain youth games). And it's been used to kick players out of tournaments since.

But different children grow at different rates, and there isn't necessarily the perfect bone-growing pattern to pinpoint every possible age of a child. And bones stop growing in adulthood, so this method simply won't tell the ages of adults. That's led to a search for other techniques.

2) Looking at chromosomes. Another possibility involved telomeres — the ends of chromosomes that get shorter and shorter every time a cell divides. Some studies have shown correlations between telomere length and a person's risk of developing cancer. But the length of telomeres isn't all that useful for predicting someone's age.

3) Looking at white blood cells. Another proposed method for verifying someone's age has been to look at a type of white blood cell called a T-cell. These specialized cells undergo an unusual rearrangement of pieces of their DNA, which changes as the cells get older. In 2010, researchers at Erasmus MC University Medical Center Rotterdam published a paper in Current Biology showing that they could use that correlation to guess someone's age. The problem? The guesses were give or take nine years — not perfectly accurate.

4) Checking out various molecules in the blood. Another method is to examine a range of molecules in the blood, such as some amino acids, which can vary with age. A 2013 study of more than 6,000 people published in the International Journal of Epidemiology found 22 such molecules in blood that correlated with age. It wasn't really accurate enough to get an impressive estimate of how old someone is. But it's possible that could improve with further research.

A more promising method? The epigenetic clock

Within the past few years, several papers have proposed a method that narrows the age estimate of a person even further. These researchers look at molecular markers called methyl groups that can get added or removed from DNA and help determine how your genes get used. (The study of these and similar DNA markers is called epigenetics and is currently a very new and active field of study.)

It turns out that some of these methylation sites can steadily accumulate or decrease in specific ways as a person ages. This can be used as an "epigenetic clock."

"I call it the clock because it is so accurate that it deserves that label," says Steve Horvath, a professor of human genetics and biostatistics at UCLA. Horvath was part of the group that in 2011 published a paper in the journal PLOS ONE showing the first good use of an epigenetic clock. It examined saliva samples to predict someone's age and was 5.2 years off someone's actual age, on average. "I was shocked how accurate it was," Horvath says.

Likewise, in 2013, researchers published a study in Molecular Cell that used blood to create an age estimate. They examined the blood of 656 people aged 19 to 101 to create a model of more than 450,000 possible markers. As they've continued to work on the model, the researchers can now guess someone's age plus or minus about two or three years, says lead author Kang Zhang, of the University of California, San Diego. It takes about two hours and maybe $10 to do it.

And two other similar models have also been published since then, in 2013 and 2014, in Genome Biology.

But they all seem to have their limits. Zhang says that his method likely won't get much more accurate because of the natural variation that exists between people of the same age.

Even so, police agencies and others are interested in using his methods to develop forensic tests. "We are working with police and laboratories around the world to develop a diagnostic test such that we can go to crime scenes and using body fluids or blood to tell how old perhaps the victims or the suspect are," he says.

At the same time, researchers have been using many of these tools, such as metabolite analyses and epigenetic methylation clocks, to better understand how aging affects the body. Such knowledge might even be used someday to predict someone's lifespan. And in many cases, that could be much more handy to know than how many candles to put on your birthday cake.

Further reading: David Stipp wrote up an interesting review in The New York Times of how biological age, rather than chronological age, could be useful for predicting and preventing disease.

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