Aging: The Difference Between Chronological and Biological Age

You might think you’ve got a handle on your age based on how many birthdays you’ve celebrated. But, well, your body might tell a different story.

Chronological age is simply a count of the years you’ve been alive. Yet, there’s also your biological age, which gauges how much your body has endured at the cellular level.

Factors like genetics, lifestyle choices, and medical background can make you feel older or younger than your years. It’s a little confusing, right? These measures aren’t synced up all the time, according to Dr. Douglas Vaughan, who heads the Potocsnak Longevity Institute at Northwestern University.

“When you have a birthday, that number goes up no matter what,” Vaughan shared. “That’s the universal truth we can’t escape.”

In contrast, biological age, or epigenetic age, pertains more to internal changes that happen as you age.

Researchers have devised various ways to estimate biological age—like epigenetic clocks that analyze DNA changes. These clocks started in clinical settings but are now becoming accessible through consumer test kits.

“Now we have plenty of tools offering a glimpse into an individual’s biological age,” Vaughan remarked. “This field is really taking off as we discover new ways to gauge biological aging.”

The growth of the longevity sector reflects in the biological age testing market, which was valued at $1.28 billion in 2024 and is expected to reach $3.09 billion by 2033, as per Dataintelo.

Various biological age test kits are available online, costing anywhere from $299 for saliva tests to $499 for blood tests. However, they vary in accuracy and what health information they provide.

Vaughan cautions, “We’re not ready to treat these tests as definitive for the general public. The data quality can be quite uncertain.”

Understanding Epigenetic Clocks

Epigenetic clocks function through DNA methylation, which Daniel Belsky, an associate professor at the Robert N. Butler Columbia Aging Center, describes as a dimmer switch that alters gene activity.

However, Belsky warns that these clocks are mostly statistical tools with no clear biological references, leading to variations in their outcomes.

For instance, a clock known as PhenoAge measures biological age, while another called GrimAge gauges mortality risk within a year.

Belsky contributed to creating DunedinPACE, which assesses the speed of biological aging. It’s similar to a speedometer—it indicates how quickly you’re aging.

With DunedinPACE, a score of 1 represents average aging speed, whereas a score of 2 indicates a lifespan accelerating twice as fast. Scores below that imply slower aging, potentially even reversing aging.

“Most people score between 0.5 and 1.5,” Belsky noted. A small difference—like aging only 11 months for every year lived—can be significant.

Biological Age as a Moment in Time

Steve Horvath from UCLA helped create the first epigenetic clock back in 2011. Despite holding a U.S. patent, he admits, “No one really uses this clock—not even me. It just wasn’t good enough.”

However, his preliminary work laid the foundation for current epigenetic clocks, including the Horvath Clock, which can evaluate different tissues and cell types, patented in Europe and China.

Horvath also developed PhenoAge in 2018 and GrimAge in 2019, with many over-the-counter tests relying on these markers.

While the main utility of these clocks is still in labs, Horvath hopes that such tests will soon become routine in medical check-ups.

“The tests aren’t ready for widespread use because clinical validation is lagging,” he explained. “Yet serious researchers are testing to find their potential benefits.”

Biological age represents more than just a figure, says Christopher Hine from Cleveland Clinic’s cardiovascular and metabolic sciences department.

“It relates your overall health—like metabolic function and bone density—to how many years you’ve lived. It’s reminiscent of the difference between lifespan—the total years lived—and health span—the years lived without chronic issues,” Hine elaborated.

A biological age test acts as a snapshot, according to him. For example, having a cold might skew the results.

“Testing just once won’t tell you much—you need a series of snapshots to understand your aging pattern better,” Hine added.

A Broader View of Biological Age

Though Hine believes biological age tests will play a bigger role in healthcare soon, he warns that they’re not meant for diagnosing health issues just yet. Testing on impulse can carry risks.

Someone who sees themselves as healthy might be surprised to discover their biological age is five years older than their chronological age, prompting drastic lifestyle changes.

“This could lead to individuals obsessing over lowering that biological number,” Hine noted. “It’s better to consult a healthcare professional rather than self-diagnosing or self-medicating.”

According to Vaughan, chemical changes to DNA can be somewhat flexible. For instance, research suggests that smoking can speed up biological aging, while daily multivitamins may help slow it down in older individuals.

Belsky advises consumers to approach test results carefully, noting that different clocks can yield conflicting readings.

“These are research tools providing additional insights for those curious,” he remarked. “Some are better predictors of health outcomes than others.”

Vaughan emphasizes that while chronological age is vital in predicting diseases, biological age might offer even clearer insights.

“It enhances our understanding of what factors increase disease risk as we age,” he said. Yet solely relying on biological age can be misleading.

“Don’t jump to conclusions about a positive biological age result,” Vaughan cautioned.

“Finding out your biological age is 10 years younger than your chronological age makes for great conversation, sure. But what about your cholesterol? What’s your blood pressure? Those markers are crucial to consider too.”