Eventually, ?old age? became a last resort phrase to describe an unknown cause of death. Or it became useful where a person may have died from a number of complications, but where it was not practical or ethical to order an autopsy to find the precise underlying cause of death.

In modern medicine, "dying of old age" is no longer considered a legitimate cause of death. As of 2022, the term was officially removed from many international health documents and replaced with the more precise "aging-associated biological decline in intrinsic capacity." When people use the phrase, they usually mean that an elderly person died from a natural, expected complication of being frail, such as a peaceful heart failure or "slow-onset" organ failure, where no single acute trauma was the culprit. Aging itself doesn't kill; rather, it causes the body's systems (immune, cardiovascular, etc.) to lose their "reserve," making a minor infection like a cold or a small fall potentially fatal. Doctors now strive to identify a specific underlying cause—like pneumonia, stroke, or cardiovascular disease—even in the very elderly, to better understand the pathology of aging. Essentially, "dying of old age" is a social term for a natural death where the body simply "wears out" and loses the ability to fight off the standard stresses of life.