Romans 5:12 tells us that:
Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned.
What this verse is trying to tell us is that death is an unavoidable consequence which came into the world through sin.
While it's true that the devil isn't meant to have power over believers, certain actions can unfortunately give him a right to touch us. Sometimes, death can come due to disalignment – when people step outside of God's will. Making choices that go against God's plan leaves us vulnerable and susceptible to the attacks of the wicked one.
When we live within God's will, Satan simply can't touch us. It’s not primarily our prayer life that keeps us alive, but rather our position in Christ. We are either in Christ or outside of Him. And for our own good, staying firmly in Christ is paramount. For some, death might even come as a form of judgment. Remember, God is merciful, but He is also a consuming fire.
God values order and dislikes anything that disrupts it. When someone passes away, we shouldn't be too quick to question God. We often don't know the kind of life that person might have lived. Some people might even die due to a lack of proper care for their health. The Bible reminds us that we will give an account of how we used our bodies. This includes everything in general, even our physical health.
There are some things that are left in our hands. In the small ways we can, we should make sure to eat healthy and exercise regularly. While these actions don't guarantee us a hundred percent long life, they certainly increase our chances for a healthier, fuller one.