If you have wondered how you can tell what your biological age is, there is one mildly inconvenient answer: no single number can inspect the machinery, stamp a date on it and declare you 57 despite the candles insisting otherwise.
Biological age is an estimate of how the body appears to be ageing internally, and it makes most sense alongside measures such as waist size, blood pressure, blood sugar, body composition, fitness and lifestyle — not as a replacement horoscope for the bathroom scales.
That distinction matters in England, where the 2024 Health Survey for England found that 30% of adults were living with obesity and 66% were classified as overweight. More strikingly, 70% had a waist-to-height ratio above the recommended 0.5 level, indicating increased or high central adiposity.
Those figures also expose the weakness of asking one measurement to do everybody’s homework.
Biological age is not one magic test
Chronological age is magnificently uncomplicated. Subtract your year of birth from the current year and, barring an impending birthday, the arithmetic is largely done.
Biological age is different. Researchers use health and physiological measures to estimate how the body is functioning relative to the ageing process. Factors studied include weight, blood pressure, blood glucose, inflammation, physical activity and other lifestyle behaviours.
Some research also uses molecular markers of ageing. A 2025 JAMA Network Open study cited in the supplied evidence linked long-term obesity with molecular ageing signatures including epigenetic changes and telomere shortening.
That makes biological age potentially interesting, but it should still be viewed as an estimate rather than a clinical verdict engraved on stone tablets.
BMI is useful — but it cannot see everything
Body Mass Index remains an extremely quick screening calculation: weight relative to height. Its great advantage is that practically anyone can calculate it. Its drawback is that practically anyone can then ask it questions it was never designed to answer.
BMI cannot distinguish muscle from fat and does not identify where fat is stored.
Recent research reported from Italy underlines the problem. In a comparison between BMI classifications and body scans assessing fat, muscle and bone, 34% of patients classified as obese by BMI were reportedly misclassified and belonged in the overweight category, while 53% classified as overweight were in the wrong category.
Jordan Chadwick, Managing Director at Tonic, puts the limitation neatly: “Two people can have exactly the same BMI and be in very different places health-wise. One person may have more muscle, better fitness and stable blood sugar, while another may be carrying more weight around the waist and have a higher level of metabolic risk. That is why looking only at height and weight can be too blunt.
“Biological age is interesting because it moves the conversation away from simply asking, ‘What do you weigh?’ and towards asking, ‘How well is your body functioning?’ That is a much more useful question for long-term health.”
Waist-to-height ratio adds another useful clue
Waist measurement supplies something BMI cannot: information about where excess fat is concentrated.
Central adiposity matters because fat carried around the abdomen is associated with metabolic risk. That makes waist-to-height ratio a useful companion measurement rather than a rival attempting to knock BMI from its perch.
The NHS threshold cited in the source is straightforward: waist circumference should generally be less than half your height.
A 2025 Scientific Reports study involving 22,921 adults adds another strand. Higher BMI and waist-related body-shape measures were significantly associated with biological age acceleration.
Association does not prove that one measurement itself causes accelerated ageing. It does reinforce the broader point: body composition and metabolic health deserve rather more attention than the number on a set of scales.
Can you improve your biological age?
The more useful part of the biological-age conversation is not discovering that an algorithm thinks you are four years older than expected and spending Tuesday sulking about it. It is identifying modifiable health factors.
Six habits stand out from the evidence and advice supplied.
1. Work towards a healthier weight sustainably. Long-term obesity has been associated with markers of biological ageing. Severe restriction and short-lived weight loss are hardly victories if the result disappears six weeks later.
“A healthier weight can reduce strain on the body, but the route matters. If someone loses weight in a way that is too restrictive or unsupported, they may struggle to keep it off. Sustainable change usually comes from a plan that protects nutrition, supports movement and fits real life.”
2. Measure your waist as well as your weight. Waist-to-height ratio can reveal central adiposity that BMI misses.
3. Move regularly. Physical activity supports cardiovascular fitness, insulin sensitivity, strength and mood. Its value extends rather further than the calorie display on a treadmill.
4. Build and retain muscle. Muscle contributes to strength, mobility and metabolism, while BMI cannot distinguish a muscular body from one carrying the same weight predominantly as fat.
5. Protect your sleep. Sleep affects appetite, mood, recovery and the ability to make reasonably sensible decisions before coffee has restored diplomatic relations with the world.
6. Treat psychological health as part of the programme. Shame, anxiety around food and fixation on measurements are poor foundations for sustainable behaviour.
The best biological-age test is a broader health picture
There is an understandable appeal in turning ageing into a score. Numbers feel precise. Human biology has not always had the courtesy to cooperate.
BMI can flag weight-related risk. Waist-to-height ratio can add information about fat distribution. Blood pressure, glucose, fitness, muscle mass, sleep and lifestyle provide further context. Biological-age estimates attempt to bring parts of that picture together.
None deserves to become a solitary verdict.
Chadwick makes the more important distinction: “Losing weight is never just about making the number on the scale smaller. The real question is whether the body is becoming healthier as a result. Are blood sugar levels improving? Is the person moving more easily? Are they sleeping better? Do they have more energy and more confidence in their routine?
“That is why quick fixes can be so misleading. Someone can lose weight quickly but still feel exhausted, unsupported and unable to maintain the change. A healthier approach looks at what is happening underneath: metabolism, movement, nutrition, sleep and the mental side of weight loss.”
So if the question is how old your body really is, resist the temptation to make one test the oracle. Measure the things that matter, follow their direction over time and improve the habits capable of moving them.
Your birthday is fixed. Your health trajectory has rather more room for negotiation.
