Your Birth Date Is Arbitrary — It's Your Biological Age That Matters

Though your chronological age records the passage of time, your biological age records what’s happening inside you.

Old Woman With Zipper Revealing Young One Underneath.

In a study published in the journal Molecular Cell, researchers discovered rapid aging in HIV patients. Biologist Trey Ideker and his team at the University of California, San Diego made this discovery, finding that these patients were susceptible to age-related diseases such as osteoporosis, heart disease, and dementia five years earlier than their non-infected peers. Researchers aren’t sure whether it is anti-retroviral drug treatments or the virus itself that causes this. But some aspect seems to speed up their biological age. So what is one’s biological age, and how is it different from the chronological kind?


Our biological age is how our bodily tissues, systems, and organs grow older. Put another way, it is how close or far away our organs are from age-related conditions. One’s biological age is a far better indicator of their health than their age in years. This is usually determined by looking at a person’s genetic code. There can be dramatic discrepancies, such as one being a full decade older or younger biologically, than they are chronologically. One thing geneticists look at is telomeres. These reside at the ends of chromosomes. They protect the ends from fusion with other chromosomes and from deterioration. Telomeres affect how long cells live and when they die.

A telomere’s length helps determine when a cell meets its end. A bead falls off the end of a chromosome every time a cell divides. The longer one lives in years, the shorter the length of their telomeres. Those with telomerase mutations or shorter telomeres are more likely to die early, either by developing a neurodegenerative disorder such as Alzheimer’s or from a serious illness. Researchers aren’t sure, but some evidence suggests that leading a healthy lifestyle may help to maintain the length of telomeres. 

Another important indicator is methylation of DNA. This has to do with epigenetics, or adaptations to the environment which are then written into our genes. Methylation are labels that tell cells whether they should lock genes in the on or off position. Pivotal for the developing embryo, methylation aids in the process of cell differentiation. This is the reason why a heart cell and brain cell can be coded the same but function differently.

A chromosome with telomeres in red.

In 2013, UCLA geneticist Steve Horvath decided he wanted to see if methylation could predict a person’s age using tissue and cell samples. The idea was to determine one’s biological age and in that, their susceptibility to age-related diseases. He took 8,000 samples from 51 different kinds of cells and tissues. Soon, Horvath discovered that one’s chronological age was often very different from their biological one.

Not all organs in one’s body age the same age. Some grow older more quickly than others. Women will be saddened to know that breast tissue is one of the most mature parts of the human body. A woman’s breasts can actually be up to three years older than the rest of her. Breast cancer is so prevalent, and Horvath believes that biological age has something to do with it. If cancer tissue is present, the adjacent healthy breast tissue will be about 12 years older than a woman’s chronological age, or more. Horvath believes these insights will lead to better diagnostics and treatment options for breast cancer, among other diseases.

DNA methylation

In another experiment, Horvath took tissue samples from childhood cancer patients. Some brain cancer samples were found to have a biological age of 80 years. For some good news, human heart tissue was found to be younger than many others in the body. Stem cells are often recruited to help create new cardiac muscle, knowledge that someday may help us combat cardiovascular disease in an entirely new way.

Another study looked at the elderly and their biological age. Those who had poor functioning were found to be biologically older, while those with better functioning were biologically younger. Now with Horvath’s studies in hand, researchers can look into what pathways biological aging takes, and discover new entry points and therapies to treat age-related diseases. Researchers might even find a way to undo aging itself. According to Horvath, "The big question is whether the biological clock controls a process that leads to aging. If so, the clock will become an important biomarker for studying new therapeutic approaches to keeping us young."

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