Can Training Make Twins Age Differently?

Is age just a number?

Chronological age is simple: it counts the time since you were born. Twins have the same birthday and therefore the same chronological age.

The calendar does not, however, say how fast someone climbs stairs, what their fitness level is, or what their body composition looks like. Two people of the same age can differ clearly in such functional and biological measures.

That does not mean chronological age is unimportant. It means ageing can be described on several levels — and that each level requires its own measure.

What is biological age?

"Biological age" is used for various models that try to summarise how the body functions or changes compared with chronological age. The models can be built on entirely different data: physical function, blood markers, organ measurements, or molecular markers.

There is therefore no universal biological age number that all researchers measure in the same way. Fitness, muscle strength, and body composition can describe functional aspects of ageing, while DNA methylation age is a specific molecular research measure.

A person may rank better on one measure and worse on another. The article therefore uses the concept as a cautious entry point, not as a diagnosis or precise age calculation.

Why twins are particularly interesting

Identical twins share essentially the same DNA sequence and often much of their early environment. When they later develop different activity habits, researchers gain an unusual opportunity to compare people with a very similar genetic starting point.

The difference between co-twins can make certain lifestyle associations more visible than in a comparison between unrelated people. At the same time, twins do not live identical lives. Work, illness, sleep, diet, stress, and many other factors can develop differently.

The twin design therefore reduces some uncertainties but does not remove them. Nor does it turn an observational study into a randomised experiment.

What the research actually shows

The 2022 review summarises findings from 17 rare Finnish identical twin pairs with long-term differences in activity levels. In the reviewed pairs, the more active twins had higher physical capacity and less total body fat, abdominal fat, and liver fat.

The FITFATTWIN study from 2015 examined 10 male identical twin pairs aged 32–36. In this small, selected sample, the more active twins had better fitness, less body fat, and differences in certain glucose-related markers, among other things.

The results show that genetically very similar people can differ in several measurable characteristics when their long-term activity habits differ. They do not show that researchers measured a single biological age number or that activity alone created every difference.

What the research does not show

The studies do not show that exercise definitively slows ageing or makes a person biologically younger. Nor do they show that a less active twin automatically ages faster.

The verified sources do not support writing that exercise directly prevents dementia, cancer, or cardiovascular disease in these twin pairs. They also did not measure a complete chain from exercise to inflammation, mitochondria, and a longer life.

A strong visual or functional contrast between a pair is easy to interpret as a before-and-after experiment. Research is more cautious: it describes observed differences in small and specially selected groups.

Muscle mass, fitness, and functional ageing

For the reader, functional ageing is often more concrete than an abstract age number. Can the body carry, walk, rise, recover, and do what matters in daily life?

The verified twin sources support differences in physical capacity and body composition between some more and less active co-twins. The FITFATTWIN source also supports a difference in measured fitness in its small sample.

The sources are not sufficient, however, for a broad article claim about sarcopenia, independence in old age, or the idea that a certain amount of exercise preserves muscle mass for everyone. Such claims require supplementary age- and exercise-specific sources.

Lifestyle, epigenetics, and biological markers

Epigenetics is a possible part of the broader story about how genes and environment interact, but this article does not make it the main explanation. The verified Finnish study examined a specific DNA methylation age measure.

When researchers compared 16 older twin pairs with long-term differences in activity levels, they found no clear difference in DNA methylation age between the more and less active co-twins. The authors concluded that leisure-time physical activity in adulthood had at most a minor effect on acceleration in this particular measure.

That result is important because it slows an overly simple narrative. Twins can differ in fitness and body composition without the same difference appearing clearly in every molecular "age clock".

The fun truth about ageing

Twins perhaps have the most detailed timeline of anyone: who got the first grey hair, who started holding their phone further away, and who still insists the muscle soreness is "only temporary".

Research does provide some support for what many twins already notice — the same birthday and very similar DNA do not guarantee identical function or body composition throughout life.

But it gives no scientific right to appoint the "younger" twin after a run. The calendar is shared. The bodies' stories are more nuanced.

What does this mean for twins?

If you and your twin have different activity habits, it can be interesting to follow how you feel and what you can do — without turning the comparison into a diagnosis or competition.

Research does not say that one of you has a certain biological age. It says that long-term different lives can coincide with measurable differences, even in genetically very similar people.

The most useful perspective is therefore not "who is ageing fastest?" but "which aspects of function and health can we track in a safe and meaningful way?"

The TwinPare perspective

At TwinPare we are fascinated by the moment when similarity meets life experience. Twins can start with nearly the same genetic blueprint and still become less biologically alike over time.

That is a hopeful thought, but not a promise of control. Genes, environment, illness, chance, and lifestyle habits interact in ways no single age measure can fully capture.

You cannot stop time. Research does suggest, however, that functional characteristics and body composition cannot be read solely from a birth date or a DNA sequence.

Source and limitations

The article uses three verified sources. The 2022 review summarises findings from 17 long-term activity-discordant identical twin pairs. The FITFATTWIN study from 2015 covered 10 male identical twin pairs aged 32–36. The epigenetics study from 2019 analysed DNA methylation age and a small co-twin group of both identical and fraternal pairs.

The sources support cautious claims about differences in physical capacity, fitness, body composition, certain glucose-related markers, and the specific methylation age measure.

They do not prove that exercise stops or reverses ageing, that a twin receives a certain biological age, or that all long-term active people get the same result. The small, observational, and selected samples limit generalisability.

Claims about sarcopenia, inflammation, mitochondrial function, dementia, cancer, cardiovascular disease, and precise ageing rate require additional verified sources. They are therefore not used as direct conclusions in the article.