My answer would be that there is no key, to staying healthy.
Everything is dependant, on everything else. We need a good dose of luck and strong genetic background, too.
I find that age is "Inevitable, Mr. Anderson".
Avoid saturated fats, salt and processed sugar. Do exercise, like walking. We are built as creatures, to walk. Our ancestors roamed the globe, by walking. It is gentle on the body and it's not by accident that an hour's walk is called a "constitutional" one.
My belief is that we need both a healthy mind and body; as do and have, many others. Meditation helps, as does keeping one's focus on today and not tomorrow.
Avoid saturated fats, salt and processed sugar.
Early to bed and early to rise; an apple a day, etc. These are all good maxims for avoiding sickness. There's many of 'em. They're all relevant and part of staying healthy.
Avoid saturated fats, salt and processed sugar ....... and get some exercise!!