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".
"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 .....