“AGE is an issue of mind over matter. If you don’t mind, it doesn’t matter.”
This nugget of wisdom, often attributed to Mark Twain, has been turned into many an inspirational internet meme over the years. As a 51-year-old who is starting to feel the gathering momentum of the inevitable slide, it strikes me as little more than a platitude that makes people feel better about getting old.
But according to a growing body of research, there is more to it than that. Subjective age – how old we feel – has a very real impact on health and longevity. People who feel younger than their years often actually are, in terms of how long they have left to live.
The question of what controls our subjective age, and whether we can change it, has always been tricky to address scientifically. Now, research is revealing some surprising answers. The good news is that many of the factors that help determine how old we feel are things that we can control to add years to our lives –and life to our years.
We have known for a while now that simply counting the number of years someone has been alive isn’t necessarily the most accurate way of gauging longevity. Biological “ageing clocks” measure various markers in the body to see how far along the physical ageing process we are (see “Old bones?“). But we also know that physical ageing is not the be-all and end-all. Gerontologists recognise that just as we can make generalisations about the ways that physical ageing affects our bodies – a 60-year-old will almost certainly show more signs of physical decline than a 30-year-old – there are some predictable psychological changes that come with age, too.
In the late 1990s, Laura Carstensen, a gerontologist at Stanford University in California, measured how human psychology typically changes as we age. Her work has shown that young people, for whom time seems unlimited, are motivated to pursue knowledge about the physical and social world – to explore and make new connections. As a result, they tend to be more enthusiastic, outward-looking and sociable than their parents and grandparents, but also more superficial, impulsive and emotionally fragile.
Older people, meanwhile, feeling that they have fewer years left to play with, turn away from exploring and concentrate on finding meaning, emotional intimacy and sharing the wisdom of their years.
Just a feeling?
Even within this general psychological trajectory, however, subjective age varies considerably. This isn’t terribly surprising: we all know people who are young at heart and young fogeys who think and behave older than their years. Intriguingly, though, studies suggest that being young at heart is seriously good for you. A lower subjective age is correlated with better health, longevity and general well-being, while people with a greater subjective age have higher levels of inflammation, a marker of general ill health, and older-looking brains.
“A lower subjective age is associated with better health and well-being”
A 2018 paper by Antonio Terracciano at Florida State University and his colleagues, looked at data from three studies following more than 17,000 people for up to 20 years. They confirmed that subjective age isn’t just a feeling, but also a pretty accurate predictor of health. “People who feel younger live longer. Those who feel older have a shorter lifespan,” he says.
So you can get a rough idea of your longevity by figuring out your subjective age. The trouble is that it’s not as simple as asking people how old they feel, says Maria Mitina, a biologist at Hong Kong-based biotech company Deep Longevity who is working on the problem. Subjective age can fluctuate widely depending on mood and circumstances, so people’s answers may not reflect how old they feel most of the time.
Each of us has a “baseline” that we consistently return to and which may or may not match up with our age in years or our position on the psychological timeline, says Mitina. In this respect, subjective age is like another important quality-of-life measurement, happiness. People’s self-reported happiness levels vary greatly from day to day and even hour to hour, but an individual’s happiness tends to fluctuate around a characteristic baseline. Somebody who is temperamentally cheerful can have bad days, but will always gravitate back towards this happy medium.
Because of short-term fluctuations in subjective age, simply asking someone “How old do you feel?” isn’t a particularly reliable guide to their baseline subjective age. “It is not a constant variable: maybe today you feel happier and younger, but in two weeks, you are unhappy and your subjective age will change,” says Mitina.
How to sort a baseline subjective age from all the fluctuations? Alex Zhavoronkov, founder of Deep Longevity and a researcher at the Buck Institute for Research on Aging in California, wondered if artificial intelligence could help. He had already used AI to discover new markers of biological ageing. According to Zhavoronkov, such biological clocks are one of the most important recent advances in ageing research. However, up to now the psychological side has been overlooked.
The chief benefit of using AI is that it can spot patterns in large data sets that aren’t discernible to humans, allowing it to link subjective age to factors that appear to have little to do with it. The data set Zhavoronkov, Mitina and their colleagues chose came from a project called MIDUS (Midlife in the United States). This was a research programme spanning 20 years run by the US National Institute on Aging, which was designed to understand how behavioural, psychological and social factors influence health and well-being with age. The hope was that AI would allow the team to develop a psychological ageing clock like the biological one.
For MIDUS, thousands of people in the US aged 25 to 75 were interviewed, with the same 7100 individuals – dropouts and deaths aside – taking part in the 1990s, 2000s and 2010s. An extra 3500 volunteers were added in the mid 2010s. Each time, volunteers were asked more than 1000 questions about all aspects of their lives, including their physical and mental health, well-being, personality, beliefs, social lives and sex lives. Some questions, such as “How old do you feel most of the time?” and “If you could be any age, what would it be?”, were directly aimed at measuring subjective age. Others quizzed them on less obviously age-related aspects of their physical and mental health, beliefs, personality and lifestyle choices.
The first step for Zhavoronkov and his colleagues was to design AIs to comb through MIDUS questionnaires and then train them to accurately predict each individual’s chronological and subjective age from their answers.
After training them on more than 10,000 questionnaires, the researchers say they have cracked it. “We developed two psychological ageing clocks,” says Mitina. The first converts an individual’s answers into an accurate estimate of their chronological age, providing further evidence that people’s psychology really does follow a predictable pattern as they get older.
“Positive attitude is more strongly associated with long life than any biomarker”
The second, which they called SubjAge, spat out an estimate of how old people perceived themselves to be based on their answers to questions that didn’t directly ask them about subjective age. These estimates could then be checked against MIDUS questions that were designed to get an estimate of subjective age, such as: “Imagine you could be any age; what age would you like to be?”
The team then validated both models against the MIDUS answers of a further 2500 people and found that both were accurate to within seven years. That is about par for the course in ageing clock research and is good enough to be medically useful, says Mitina, but it could be better. The best biological clocks are accurate to within two years. She and her colleagues are working on adding a biomarker found in blood that they think could tighten up the estimate.
Already, though, the researchers reckon their model is good enough to allow them to identify behavioural and lifestyle factors that were both predictive of subjective age and, crucially, modifiable.
Perhaps unsurprisingly, by far the biggest influence on SubjAge is physical health. Two of the top three predictive questions are: “Does your health limit your ability to do vigorous physical exercise such as running or heavy lifting?” and “Are you taking prescription medication to manage your blood pressure?”
Less predictably, perhaps, the second-most influential factor in people’s SubjAge is how satisfying they expect their sex life to be in 10 years’ time. The effort they put into their current sex life is also in the top 10 (as the saucy joke version of the old saying goes, you are only as old as the person you feel).
“If you are very happy with your sexual life then you are psychologically younger”
Other contributions to SubjAge aren’t obviously connected to youth at all. One of the top 25, for example, is how much people feel they contribute to the well-being of others.
Ultimately, says Mitina, the goal is to turn these findings into lifestyle advice to help people feel more youthful and even live longer. The analysis showed that a 60-year-old with a SubjAge of 65, for example, is twice as likely as a 60-year-old with a matching SubjAge to die from any cause at any given age thereafter.
The magnitude of the effect is surprisingly large, says Mitina. “I think this is the most powerful outcome from our experiment – that higher subjective age doubles mortality risk.”
As for the kinds of lifestyle changes to make, the fact that the biggest influence on how old we feel is physical health could give people concrete actions they can take to get their SubjAge down, such as exercise to improve fitness and dietary changes to improve blood pressure.
Going out to explore could also pay dividends. Several questions probe a person’s attitude towards ageing as well as their relationships, community involvement and personality. In all cases, being more open and optimistic helped. People who felt positive about ageing and who rated themselves as extroverts had a lower SubjAge, for instance. “We can ask people to be more open – to new people, new knowledge or new experiences. Push people to be more sociable,” says Mitina.
Older, wiser, sexier
This chimes with research on centenarians showing that one thing they have in common is an optimistic and gregarious nature. Positive attitude is much more strongly correlated with long life than any biomarker, according to Kaare Christensen, who runs the Danish Aging Research Center at the University of Southern Denmark.
Others may want to focus their energies closer to home. “It looks like if you are very happy about your sexual life, then you are psychologically younger, so investment in closer relationships can make you younger and improve well-being,” says Mitina.
If answering 1000 questions to find out your subjective age feels like a poor use of time you won’t get back, the good news is that you don’t have to. Mitina and her colleagues have whittled the questionnaire down to just 15 questions, none of which ask directly how old you feel, and created a website where anybody can get a rough and ready estimate of their subjective age (see app.young.ai/psychoage). I did it and discovered that – depressingly – I feel exactly as old as I am right now.
I am 51; I feel 51; my subjective age is 51. I am putting this finding down to lockdown-induced restrictions on social contact and exercise. On the plus side, if living to a ripe old age requires more friendship, better sex, new experiences and a healthier attitude to ageing, count me in. 40, here I come!
There are various ways of measuring our progress along the pathway from cradle to grave. The most obvious is chronological age, which is simply how many calendar years we have on the clock.
A more accurate measure, however, is biological age. This treats ageing as a malleable, but fairly predictable, process of biological decrepitude, and uses various biomarkers – such as metabolism and genetics – to assess how far we are through it. There is also immunological age, which measures how youthful our immune systems are.
These “ageing clocks” spit out a number in years that is an estimate of where we stand in relation to an average human ageing at an average rate. It can be higher or lower than our chronological age, sometimes by more than a decade. And, crucially, it can go down as well as up, due to lifestyle changes such as exercising more or drinking less. Age really is just a number – but not necessarily the number of candles on your birthday cake.
Feeling young in a time of covid-19
Given that feeling young requires both adventure and socialising, you might think that the lockdowns and social distancing of the covid-19 pandemic are making us all – young and less so – feel ancient.
Antonio Terracciano, a gerontologist at Florida State University, thought so too, but when he did some research, he found the exact opposite. Most of the 3738 US adults he surveyed reported feeling younger in March and April 2020 than they had in January and February, before the virus took off in the US. “It’s counter-intuitive, but this is what we find,” he says.
One possibility is that feeling younger made people feel less vulnerable in the face of a disease that was seen primarily as a threat to older people. This is already thought to be a factor in subjective age, says Terracciano. “It is a reflection of psychological distancing from a societal negative stereotype of ageing as associated with disease and death,” he says.