Your body is younger than you are
A large part of your body is only a few weeks old. Your skin renews itself roughly once a month. The cells lining your gut last only a few days. Red blood cells live for about four months before they are sorted out and replaced. While you read this paragraph, your body is making millions of new cells.
So ageing is not simply a matter of piling up years. Your body does not pile anything up; it keeps swapping things out. Ageing begins where this renewal slows down and the new cells no longer turn out quite as well as they used to.
This is exactly where two kinds of age part ways. Your chronological age counts the years since you were born. It is printed on your ID and is not up for negotiation. Your biological age, by contrast, describes the actual condition of your body: how well it still renews itself and how resilient its tissues are. In people born in the same year, the two can be many years apart. A long-term study from New Zealand shows how big that gap can be: among around 950 people, all born in the same year and all 38 years old when they were measured, biological age ranged from 28 to 61.
So what exactly is older about one body than another when both have lived the same number of years? To understand that, it helps to look at how renewal works and where it reaches its limits.
Telomeres: Why cells cannot renew themselves forever
New cells are created when existing ones divide. One cell becomes two, and the complete blueprint, the genetic information, has to be copied along with it. This copying is remarkably reliable, but it has a built-in limit.
Think of the little plastic tips at the end of a shoelace. They contain nothing important; they just stop the lace from fraying. Something very similar sits at the ends of your chromosomes: the telomeres. They protect the genetic information behind them and act as a buffer every time a cell divides.
With every division, this buffer gets a little shorter. At some point it is no longer enough, and the cell stops dividing. It does not die right away but stays in the tissue as a retired cell. Over the years, more and more of these accumulate. The skin gets thinner, wounds heal more slowly, and the immune system responds more sluggishly because the supply of fresh cells is drying up.
How quickly telomeres shorten, however, is not set in stone. That describes the first pillar of biological age: how much capacity for renewal your body still has.
Blood vessels: What hardly renews itself
Alongside everything that is constantly being renewed, your body contains material that is practically never replaced. The elastic fibres in your arteries, for example, are formed mostly during childhood and adolescence. Whatever you have of them, you need to look after.
A young artery behaves like a soft garden hose. With every heartbeat it stretches, absorbs the pressure and passes it on evenly. Over the years this tissue becomes stiffer, like a hose that has spent many summers lying in the sun. The heart then has to work against greater resistance, and the pressure reaches the fine vessels of the brain, kidneys and eyes with less cushioning.
You can observe the same property in your skin. Skin that smooths out immediately after a light pinch still has plenty of elastic material. If it takes a second longer, there is less of it. That gives us the second pillar of biological age: how elastic the tissue that does not renew itself remains.
What speeds up ageing
Both processes happen in everyone. The interesting question is why they run faster in some people than in others. Three influences play a particularly important role.
The most important is inflammation you do not notice. Unlike a cold, there is no fever and no pain, just a slightly raised level of inflammation that persists for years. It forces the tissue into constant repair and therefore into more cell divisions than necessary. Lack of exercise, poor sleep, excess weight and an unbalanced diet can keep this smouldering fire going.
The second is chronic stress. Your body's alarm system is built for short-lived dangers. If it stays switched on for months, a protective mechanism turns into a burden that pushes inflammation levels up even further.
The third is damage from outside. Smoking, alcohol and a lot of UV radiation attack cells and their genetic information directly. The body repairs much of this damage, but not all of it.
What your body can do about it
If that were all, this would be a fairly depressing topic. But it is only half the story, because many of these processes can also be pushed in the other direction.
Regular endurance exercise keeps exactly the blood vessels mentioned above supple, and its traces can be followed all the way down to the level of the telomeres. Read more here (Article: Endurance). Muscle strength builds a reserve you will draw on decades later. And during deep sleep, repair processes run for which there is no time during the day.
Then there are influences that are harder to explain but still work reliably. People with a stable social circle live considerably longer on average. So do people who regularly do something that matters to them. The exact pathways are less clear than with a cigarette, yet the link is among the most robust that research knows of.
How the BioAge test estimates your biological age
These are exactly the influences the BioAge test asks about. Across five areas, exercise, nutrition, life satisfaction, consumption and self-care, you answer 25 questions about your everyday life. Each question counts as much as the evidence for its link with health and life expectancy supports. At the end you get a number in years and an assessment of where you are already doing well and where you have the most to gain.
This number is an estimate based on your answers, not a lab value. It is meant to show you where changes will pay off most.
Why it is worth it
The difference between two bodies of the same age does not appear in a single year. It is the result of many small decisions that seem trivial on their own: a staircase skipped, a night that was too short, an evening with friends instead of alone.
And that is precisely the good news. No single step will noticeably change your biological age, but no single step has to.
The processes we are talking about respond to what you do regularly, at any age.
The following articles look at the individual factors: what lies behind them, how strong the evidence is and what you can do with it.
Frequently asked questions about biological age
Can biological age be measured?
In research, yes, for example through blood values, changes to the genome known as epigenetic clocks, or telomere length. Results vary depending on the method. The BioAge test does not measure anything in a lab; it estimates your biological age from your lifestyle.
How quickly can biological age change?
Studies show the first changes at the cellular level after just a few months, for example with regular endurance training. What really matters, though, is what you keep up over years.
Isn't biological age simply down to your genes?
Genes play a role, but according to current knowledge they explain only a smaller part. The larger part is linked to lifestyle and environment. That is why the BioAge test deliberately leaves genetics aside and focuses on what you can influence.
At what age is it worth paying attention to this?
At any age. The processes described respond to regular stimuli, even if you start late.
How accurate is the BioAge test?
It provides a well-founded estimate, not a measurement. It is based on your answers and on weighting models by Bamberger and Despeghel. Depending on lifestyle, a 40-year-old can come out at anywhere between roughly 32 and 51 in the test.