As Young As Your Faith by Douglas MacArthur

People grow old only by deserting their ideals, MacArthur had written.

Years may wrinkle the skin, but to give up interest wrinkles the soul.

You are as young as your faith, as old as your doubt; as young as your self-confidence, as old as your fear; as young as your hope as old as your despair.

In the central place of every heart there is a recording chamber.

So long as it receives messages of beauty, hope, cheer and courage, so long are you young.

When your heart is covered with the snows of pessimism and the ice of cynicism, then, and then only, are you grown old.

And then, indeed as the ballad says, you just fade away.