Here's what it actually means to die 'of old age'

• "Old age" isn't really a medical cause of death.
• Queen Elizabeth's death certificate says she died of "old age" at 96.
• Older people are more likely to die as a result of common illnesses (like pneumonia) that others could survive.

Ever ask someone how their family member passed away and hear them say they simply "died of old age"?

As it turns out, that's almost never quite what's going on from a medical perspective. Aging — in and of itself — is not a cause of death. 

When most of us say that someone died of old age, what we really mean is that someone died as a result of an illness (like pneumonia) or as a result of an event (like a heart attack) that a healthy, stronger, younger person would likely have survived.

The Centers for Disease Control and Prevention keeps track of what people actually die from in the United States. In 2020, the CDC said the number one cause of death for people over the age of 65 in the US was heart disease, followed by cancer, COVID-19, cerebrovascular disease (which often leads to strokes), and Alzheimer's.

These "old age" fatalities are often quiet deaths, like what happens when you hear an older person's heart stopped in their sleep. That usually means the person had a heart attack in the middle of the night. Another example is if someone "had a bad fall, and it was just downhill from there." The person likely broke a hip, survived the surgery, but then got pneumonia and died as a result of their infection. 

Often, what claims the lives of older people can be an accumulation of things. Sometimes this is referred to as "geriatric failure to thrive," a kind of catchall phrase for elderly patients who may have a bunch of interrelated issues, including trouble with moving, eating, depression, and cognitive impairment.

"As you get older and older, you're more likely to get heart disease and cancer," Amy Ehrlich, a professor of clinical medicine at the Albert Einstein College of Medicine and a geriatric physician at Montefiore Medical Center, previously told Insider. "But we also see a lot of things like falls, where someone falls and ends up with serious trauma like a hip fracture. That's hard to recover from when you're 104."

If we don't die as a result of aging, then what the heck is aging?

Humans didn't always live reliably long enough to age. People used to die frequently long before their skin began to sag or their muscles began to wither, succumbing instead to diseases for which we now have vaccines, like tuberculosis or smallpox. Some people died from gastrointestinal infections, which can cause diarrhea. In many countries around the world, diarrhea is still a leading cause of death in kids, along with malaria, and pneumonia.

Somewhere around the 1950s (at least in the US, and other wealthy countries), we started living nearly twice as long as our ancestors had just a century before. We now spend a massive portion of our lives getting old before we die, though in recent years, life expectancy has been declining — a trend that started before the COVID-19 pandemic began.

Other living organisms don't age

What if we experienced aging without death as the final outcome — or didn't age at all? That's how some animals do it.

A 2014 study comparing the mortality rates of 46 different species found that some organisms don't age — their mortality rates stay constant from around the time they're born until around the time they die. Others enter a period of aging (like the kind most of us experience around age 65) and then come out of it, continuing their lives.

Here's a chart from that study showing what aging looks like in a modern-day human (far left). Mortality rates here are in red, fertility rates are in blue:

That sharp rise in the thin red line says: We have an incredibly long aging period.

But lots of other creatures' life spans look nothing like this. Take a look, for example, at the "immortal" hydra (second column, second row), a tiny freshwater animal that lives to be 1,400 years old. It's just as likely to die at age 10 as it is at age 1,000:

The desert tortoise has a high mortality rate in early life, after which the tortoise death rate actually declines with age. 

What does this mean for people who want to live forever?

Some scientists think we can use this knowledge of the natural world to stop aging, or at least prolong human life.

"Aging is not a relentless process that leads to death," Michael Rose, an evolutionary biologist at the University of California at Irvine and the director of its Network for Experimental Research on Evolution, previously told Insider. (Rose has published a series of papers and books on aging and evolution.) "It's a transitional phase of life between being amazingly healthy, and stabilizing."

Other researchers, like biologist and theoretician Aubrey de Grey, want to use our knowledge of other organisms to extend our lives. The proportion of people who die of age-related problems is high in wealthy countries, de Grey said in his 2014 film "The Immortalists." "It's absolutely clear that it's the world's most important problem."

But we are not hydra nor tortoises, and, for now, we can't do away with human aging. For us, aging is real, and it is long. Fortunately, many older people can still live healthy, happy lives.

This story has been updated. It was originally published in 2016.