"Die of old age" is a catch-all phrase which (having seen my mum gently fade away in October 2020) I find perfectly acceptable. Sure, a couple of contributory, long-term conditions were mentioned on the death certificate, but I knew that she had had multiple issues for years that had sent her weakening body on a downward spiral which were not mentioned. She was 91, she was overweight and she'd smoked for 70 years. All sorts of things. There was nothing controversial and when I'm asked what she died of I'm quite happy to say that it was old age, rather than list a plethora of conditions that may have contributed.