When we say an older person has "died in their sleep", what do we mean?

There is no such thing as "dying of old age", and you won't see this term written on a death certificate.

Rather, it is a colloquial way in which we refer to the death of an older person where the cause is not obvious or well-understood.

A decline in body systems

As we age, the likelihood of suffering from a range of medical problems increases as the body accumulates damage, cells no longer repair themselves, and the immune system gets weaker.

These medical issues may include heart problems, low or high blood pressure, or neurodegenerative diseases like Parkinson's disease or Alzheimer's.

And these conditions in turn affect important systems in the body, like brain function and mobility, says Dr Tuly Rosenfeld, geriatrician and Associate Professor at the University of New South Wales (UNSW).

Take, for example, a person with Alzheimer's disease. Dr Rosenfeld says the disease is associated with a difficulty chewing or swallowing, which can cause complications that lead to death.

"The reason that many older people quietly die in their sleep is because their swallowing systems don't work, they inhale food, fluid, saliva into their lungs, they catch pneumonia and they die," he says.

And these diseases also affect mobility, making older people more likely to fall, injure themselves, and end up in hospital — where they are at risk of infections they can no longer fight off.

What is written on a death certificate?

So it is never old age alone that causes someone to die. Rather, it is the increasing likelihood of complications arising from the conditions that accumulate with age.

Doctors won't always know the precise cause of death in an elderly person because they may have overlapping conditions that interact in different and subtle ways, making it difficult to determine one official cause, Dr Rosenfeld says.

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And doctors are less likely to carry out invasive testing that might uncover a specific cause in older people, compared with young, healthy patients.

"In many cases, the death certificate will only say what is the most likely cause of death without necessarily having a specific or post-mortem proven cause of death," Dr Rosenfeld says.

"Every death certificate, even when they don't know what it was, they would usually say heart attack, cardiac arrest. If it's an old person in a nursing home they probably usually say pneumonia."

Why do we age?

While a healthy diet and exercise can help keep us younger on the inside, we can't completely stop our cells from ageing as the years roll by.

Exactly why is complicated and scientists don't have all the answers.

One common notion is that there is something in our genes — a biological clock — that causes us to tick towards old age.

Another is that over time, our body and DNA accumulate damage.

Key to this is the idea of cellular senescence – a biological phenomenon in which cells stop dividing.

When senescence sets in, it limits our ability to repair tissue when we get injured. Senescent cells release substances that encourage chronic inflammation in the body, increasing a person's risk of cancer and other conditions like arthritis.

Another focus of modern research is telomeres – structures on the ends of our gene-carrying chromosomes that shorten with age. When this happens, the risk of disease seems to increase.

"The question is whether [telomeres] play a causative role in ageing, and what intervention will alter ageing in a safe way," says Dr Lindsay Wu, head of the Laboratory for Ageing Research at UNSW.

Experiments to extend telomeres in animals have been mixed – in some animal experiments, higher cancer rates were seen, but in others the process protected against some signs of ageing.

"The jury's still out on whether telomere lengthening is useful [in combating ageing]," Dr Wu says.

Can we stop the process of ageing?

Researchers all over the world are hard at work exploring ways to arrest the ageing process.

One area of particular focus is drugs that could kill off cells that have reached the point where they stop dividing.

In one study, scientists extended the lives of mice by progressively removing these senescent cells from their bodies.

Another area of interest centres around the drug metformin, which has been used to treat type 2 diabetes for decades and has been shown to keep animals healthy and extend their life.

A clinical trial called TAME (Treating Aging with Metformin) is currently testing whether metformin can act in a similar way in humans.

Dr Wu says if it is shown metformin works to maintain human health into old age, the "cheap as chips" drug will lead to huge savings in the health system.

"It's a very ambitious study, but if it works it'll be a huge public health saving."