Logically speaking, dying of “natural causes” means dying from diseases such as a stroke, heart disease, and other illnesses are the most common ones. Although, not all things are natural causes of death like car accidents.
On the other hand, dying of “old age” is quite similar. It just means someone who is old died from a disease. You can’t actually die of old age. Older people just have a higher chance of getting illnesses than younger people, and possibly dying from it.
I hope that made sense. If it didn’t, I’m sorry there’s no other way for me to explain it properly