"Cause of death:

...

II. Frailty of old age

Quelle: Die Sterbeurkunde betr. Rolf Harris des Stansamts von Windsor and Maidenhead

Rolf Harris cause of death revealed as disgraced star's death certificate shows he died 2 weeks ago | Daily Mail Online

to die of frailty of old age

I'd say to die of old age is a quite common "lay" term, if not medical.

Obviously, the old age aspect would include frailty, possibly organ failure, etc., etc.

Basically the implication is that over time, the body has worn down enough to "give up".

From google:

"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."

As an example closer to home, my mother passed away at the age of 94 in her sleep. Although she had Alzheimer's, her death was not probed, although I'm sure her heart or other organs failed. She died of old age .....

Vgl. auch

Bei der Todesart wird zwischen natürlich, nicht natürlich und ungeklärt unterschieden – beispielsweise wie folgt:

• Natürlicher Tod: Krankheit, Alter...

Totenschein: Inhalt und Kosten der Todesbescheinigung (mymoria.de)

https://www.collinsdictionary.com/dictionary/...

English translation of 'Altersschwäche'

Altersschwäche

(von Mensch) infirmity (due to advancing years); (von Tier) weakness (due to old age); (hum: von Auto, Möbel etc) decrepitude

im Totenschein meines Großvaters (gestorben 1955 im Alter von 77 Jahren) stand als Todesursache: seniler Marasmus. (Ich übersetzte es der Großmutter anhand des Duden (1953) als "Altersschwäche".)

"Simon Schwarz (1866–1940) aus Köln arbeitete in Halle als Privatgelehrter. Am 1. Oktober 1940 wurde er als Patient in die Landesheilanstalt Altscherbitz eingewiesen. Dort starb er bereits am 6. Oktober 1940, angeblich an senilem Marasmus." (Stolperstein am Haus Weidenplan 9 in Halle)

https://de.wikipedia.org/wiki/Marasmus

"In der Gerontologie bezeichnet Marasmus senilis den Abbau körperlicher Funktionen mit zunehmendem Alter und wird damit auch als Begriff für eine Todesursache verwendet. Diese Altersschwäche tritt besonders im Greisenalter auf."

https://en.wikipedia.org/wiki/Marasmus

Re. Collin's translations: I think English tends to use different words for the frailty that comes with very old age in different contexts. A lay person would indeed be more likely to say "infirmity" or "frailty" when talking about the condition of a living person, as a cause of death, "old age". Sometimes people do talk about the "old age" of a living person in the same way they would peak of a medical condition - though in my experience of polite conversation, such usage tends to be found in humorous remarks and more often when the speaker is talking about themselves than about others. E.g.... "Don't worry, I'm fine, just suffering from a little case of old age".

The person who writes a death certificate is not a lay person and would use different words, either a specific term chosen after an examination close to the time of death or post-mortem or a more catch-all term like failure to thrive.

“Old age” no longer a diagnosis as a cause of death

¨[...]

The first conference to revise the classification of causes of death took place in 1900 and revisions have occurred every 10 years since. The latest debate about replacing “old age” as a cause of death led to three different proposals. One suggestion was “senescence,” acknowledging cells undergoing lethal changes with aging. Another proposal was “decline in intrinsic capacity,” recognizing the biological, physiological, and psychological changes negatively affecting an aging person’s overall capacities. “Frailty” was yet another consideration (i.e., that reduced reserves across multiple physiological systems could be strong predictors of mortality, independent of age and gender). 

The debate is now over: As of January 2022, “old age” was removed from the ICD as a cause of death, and agreement was reached to replace it with “aging-associated biological decline in intrinsic capacity.” No more dying of old age. 

https://bcmj.org/blog/old-age-no-longer-diagn...

How “old age” was withdrawn as a diagnosis from ICD-11

https://www.thelancet.com/journals/lanhl/arti...

Dass Juristen den Medizinern Diagnosen wie eine Todesursache vorschreiben, halte ich für ein Gerücht. Es sind schon die Mediziner selbst, die es als nicht fachgerecht empfinden, mit "Altersschwäche" in den Totenschein quasi hineinzuschreiben "ich kenne keine genaue Ursache". Das englische "old age" ist ja noch unbestimmter als die Altersschwäche.