This article first appeared in Issue 13 of our free digital magazine CURIOUS.

We’re often told that “age is just a number.” After all, when “40 is the new 30”, but then “life begins at 50”, it’s easy to think that none of it means anything at all. But there have to be some biological indicators of aging, don’t there – some signs in our bodies that track the passage of time? Or is it really all in the mind?

The ticking biological clock

Human bodies don’t carry on forever. As we go through our lives, systems that once worked well start to gradually break down, and this leaves its mark. 

The complex nature of aging means it’s unlikely a single biomarker could ever truly capture all the factors.

You can’t talk about aging without mentioning telomeres. These short regions of repetitive DNA cap the ends of every chromosome in every cell. Each time a cell divides, the telomeres shorten, until they reach a point where they’re so degraded that it’s a signal for the cell to die. This helps prevent cells from hanging around long enough to build up enough mutations to cause a serious problem.

The link between shorter telomeres and shorter life expectancy was first made in research published over two decades ago. In a random sample of people aged 60 or over, those with shorter telomeres were found to be more likely to die from an infectious condition or heart disease.  

Since then, telomere length has come to be accepted as a biomarker of aging – a proxy we can measure to roughly estimate someone’s age-related state of health. However, not everyone agrees on that. 

A 2021 review summarized the conflicting viewpoints. Studies have reported that telomere length is no better a predictor than just taking someone’s actual age in years. The authors of the review concluded that the complex nature of aging means it’s unlikely a single biomarker could ever truly capture all the factors, but telomere length could continue to be useful when considered together with other indicators.

Males and females also have their own unique indicators of age – the gradual loss of the Y chromosome from some cells, and the menopause, respectively. Y chromosome loss has been linked to certain types of cancer, among other conditions. 

Aging also manifests as general wear and tear in most organs and tissues of the body. Whether one system is affected more than another will come down to the individual, but this explains why so many diseases, like cancer, dementia, and osteoarthritis are much more common in older people, and why things like eyesight and hearing can gradually decline over time.

What’s in an age?

At a conference in March 2023, Dr Julian Mutz of King’s College London discussed the findings of a study that suggested people with a history of mental illness were, in a sense, older than they should be.

“We found that, on average, those who had a lifetime history of mental illness had a metabolite profile which implied they were older than their actual age,” Mutz explained in a statement. “For example, people with bipolar disorder had blood markers indicating that they were around 2 years older than their chronological age.”

Many people start to develop gray hair long before they feel “old enough” to do so.

Another recent study found that hospitalized COVID-19 patients who had prior exposure to high levels of air pollution had poorer outcomes from the disease, with an equivalent impact on time hospitalized of adding 10 years to their chronological age.

Further research looking at the impact of COVID-19, and the stress of living through such a tumultuous time, found that teenagers' brains were maturing more quickly than they should – something generally only seen in children who have experienced chronic trauma or neglect.

There are also conditions that cause premature aging like progeria, a rare genetic disorder.

Our old friends the telomeres, like any other system in the body, can go wrong, leading to diseases with symptoms usually linked to aging, such as osteoporosis.

And for a slightly more frivolous example, many people – this author included! – start to develop gray hair long before they feel “old enough” to do so.

But there’s also no shortage of studies claiming to have found the secret to delaying aging. One well publicized example recently found that the amino acid taurine boosted lifespan in animal models. Another 2023 study showed that researchers could extend the lifespan of worms by 50 percent by manipulating the metabolism of fat by-products that build up within cells during aging. If that same effect can be achieved in humans, study author Dr Eyleen Jorgelina O’Rourke said in a statement, it could be a route to increasing “the number of years of independent healthy living for all of us.”

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And that’s not even counting the number of strange and mysterious methods favored by the super-rich to try to halt the inevitable passage of the years, many of which have a dubious basis in actual science. Employing the services of your younger relatives to provide you with transfusions of their “youthful” blood, for example, is not something we’d recommend. 

There’s even been some research to suggest gray hair can revert back to its natural color, albeit in a limited way. 

In vindication of octogenarian extreme sports enthusiasts everywhere, research has suggested that people who feel “younger” may live longer.

The point to all this is that the chronological age you are sometimes bears little relation to how you feel. After a frustrating day at work, you might quip that you've "aged about 10 years". Conversely, many an active nonagenarian has been heard to comment that they don't "feel" all their 90+ years.

Perhaps, then, the key is to be found in the mind.

“You’re only as old as you feel”

If the number of candles on your last birthday cake represents your chronological age, the age you feel inside is your “subjective age”. 

We can make broad generalizations about the psychology of aging. It’s often said that young people feel invincible, and it’s true that the young tend to be more outward-looking and impulsive. But you probably know someone who bucks that trend, whether it’s a skydiving granny or a teenaged “old soul”. 

Well, in vindication of octogenarian extreme sports enthusiasts everywhere, research has suggested that people who feel “younger” may live longer.

That was the conclusion a 2015 study of over 300 Ashkenazi Jewish people aged between 94 and 109, part of a growing body of evidence that a positive outlook can promote longevity. Another study linked optimism to an 11-15 percent increase in lifespan, while another found that a lower subjective age could be a marker of brain health.

If your approach to life can have such a strong effect on how long you live, then, you might easily conclude that age is more of a psychological than a biological phenomenon.

But it’s very easy to talk about maintaining an upbeat attitude to life – in reality, it may not be that simple to achieve. 

The authors of the 2015 study alluded to this in their paper. All of the older adults recruited for the study were assessed as having no significant cognitive impairment, but cognitive decline is an unfortunate fact of aging for many people. It’s another of those pesky biological factors we talked about before – the brain, like every other system in the body, is not immune to wear-and-tear with the passing of the years. Cognitive dysfunction, whether it be mild or a feature of something more serious like dementia, can impact mood and emotions. 

Not only are our biology and our psychology independently altered by age, but each can also work the other way, influencing our experience of aging.

Similarly, a study that used artificial intelligence to provide a measurement of subjective age based on the answers to a US-wide survey of health and wellbeing found that physical health was by far the biggest influence on people’s scores, including things like the ability to do vigorous activity. Speaking of which… the second most important factor was how respondents foresaw their sex lives in 10 years’ time.

All joking aside, this points to the vital importance of interpersonal relationships, as well as physical health. Loneliness can pose a real problem for older people. There’s a known correlation between social isolation and poor cognitive health, and yet charity Age UK estimates that 1.4 million older adults in the UK alone are frequently lonely. 

Is age biological or psychological?

On balance, it seems the best answer to this question is: both. Not only are our biology and our psychology independently altered by age, but each can also work the other way, influencing our experience of aging – and on top of that, they also interrelate with each other. It turns out aging is a complex thing.

Despite the research demonstrating the importance of subjective age, it’s unlikely doctors will stop paying attention to chronological age any time soon. It’s still one of the best indicators we have of the relative risk of many health issues and is used as the basis of successful disease screening initiatives.

But there are some small signs that the tide is turning. For example, a recent study reported a possible new way of deciding whether donor hearts are suitable for transplantation, based on a more direct measure of the health of the organ rather than just the donor’s age. Such advances could save precious organs from needlessly going to waste.

Biological aging is inevitable, but it’s not the whole story. Psychological research is showing that, in a very real way, interventions to support better mental health and wellbeing could help people wrest back some of the control from old Father Time, and maybe add a year or two onto their lives to boot.

CURIOUS magazine is a digital magazine from IFLScience featuring interviews, experts, deep dives, fun facts, news, book excerpts, and much more. Issue 16 is out now.