This new understanding of the ageing process has been decades in the making. Some of the earliest studies charting the gap between felt and chronological age appeared in the 1970s and 1980s. That trickle of initial interest has now turned into a flood. A torrent of new studies during the last 10 years have explored the potential psychological and physiological consequences of this discrepancy.

One of the most intriguing strands of this research has explored the way subjective age interacts with our personality. It is now well accepted that people tend to mellow as they get older, becoming less extroverted and less open to new experiences – personality changes which are less pronounced in people who are younger at heart and accentuated in people with older subjective ages.

Interestingly, however, the people with younger subjective ages also became more conscientious and less neurotic – positive changes that come with normal ageing. So they still seem to gain the wisdom that comes with greater life experience. But it doesn’t come at the cost of the energy and exuberance of youth. It’s not as if having a lower subjective age leaves us frozen in a state of permanent immaturity.

Feeling younger than your years also seems to come with a lower risk of depression and greater mental wellbeing as we age. It also means better physical health, including your risk of dementia, and less of a chance that you will be hospitalised for illness.

Yannick Stephan at the University of Montpellier examined the data from three longitudinal studies which together tracked more than 17,000 middle-aged and elderly participants.

Most people felt about eight years younger than their actual chronological age. But some felt they had aged – and the consequences were serious. Feeling between 8 and 13 years older than your actual age resulted in an 18-25% greater risk of death over the study periods, and greater disease burden – even when you control for other demographic factors such as education, race or marital status.

There are many reasons why subjective age tells us so much about our health. It may be a direct result of those accompanying personality changes, with a lower subjective age meaning that you enjoy a greater range of activities (such as travelling or learning a new hobby) as you age. “Studies have found, for example, that subjective age is predictive of physical activity patterns,” Stephan says.

But the mechanism linking physical and mental wellbeing to subjective age almost certainly acts in both directions. If you feel depressed, forgetful, and physically vulnerable, you are likely to feel older. The result could be a vicious cycle, with psychological and physiological factors both contributing to a higher subjective age and worse health, which makes us feel even older and more vulnerable.

Stephan’s analysis, which is now in press in the journal of Psychosomatic Medicine, is the largest study of the effect of subjective age on mortality to date. These large effect sizes demand close attention. “These associations are comparable or stronger than the contribution of chronological age,” says Stephan.

Put another way: your subjective age can better predict your health than the date on your birth certificate.

With this in mind, many scientists are trying to identify the social and psychological factors that may shape this complex process. When do we start to feel that our minds and bodies are operating on different timescales? And why does it happen?

Working with Nicole Lindner (also at the University of Virginia), Nosek has investigated the ways the discrepancy between subjective and chronological age evolves across the lifetime. As you might expect, most children and adolescents feel older than they really are. But this switches at around 25, when the felt age drops behind the chronological age. By age 30, around 70% of people feel younger than they really are. And this discrepancy only grows over time. As Nosek and Lindner put it in their paper, “Subjective ageing appears to occur on Mars, where one Earth decade equals only 5.3 Martian years.”

Lindner and Nosek also measured the “desired age” of their participants – which, to their surprise, also followed Martian time. “It keeps going up with us, and at just a slightly slower rate than how we feel right now,” Nosek said. This would seem to “support the idea that we experience our life experiences as continuously getting better, just a bit more slowly than our actual experiences,” he says. It’s not as if there is one single peak age. Again, this flip occurs in our mid-20s: 60% of 20-year-olds want to be older. But by the age 26, 70% would prefer to be younger, and from then on, most people view the recent past with the rosiest spectacles.

Some psychologists have speculated that a lower subjective age is a form of self-defence, protecting us from the negative age stereotypes – as seen in a nuanced study by Anna Kornadt at Bielefeld University in Germany.

Kornadt’s study hinged on the idea that people’s subjective age might be a multifaceted thing that varies in different domains. You may feel differently when you think about yourself at work compared with when you think about your social relationships, for example. And so Kornadt asked participants to say whether they felt younger or older than they really were in different areas of life.

Sure enough, she found that people’s subjective ages were lower when negative age stereotypes are most prevalent – such as work, health and finance – which would seem to support the idea that this thinking helps people distance themselves from the negative connotations of their age-group. Believing “I may be 65 but I only feel 50” would mean you are less worried about your performance at work, for instance. Kornadt also found that people with a lower subjective age tended to imagine their future self in a more positive light.

By protecting us from our society’s dismal view of ageing and giving us a more optimistic view of our future, this self-defence could, in turn, further explain some of the health benefits of feeling younger than you really are.

Despite these advances, scientists are only getting to grips with their potential implications, though it is certainly possible that future interventions might try to reduce participants’ subjective age and improve their health as a result. In one of the few existing studies, elderly participants in a fitness regime enjoyed greater strength gains if the experimenters praised their performance relative to other people of their age.

And given its predictive power – beyond our actual chronological age – Stephan believes that doctors should be asking all their patients about their subjective age to identify the people who are most at risk of future health problems to plan their existing health care more effectively.

In the meantime, these findings can give us all a more nuanced view of the way our own brains and bodies weather the passing of time. However old you really are, it’s worth questioning whether any of those limitations are coming from within.

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David Robson is a science writer based in London, UK. He is d_a_robson on Twitter.

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