But my opinion is that you can do it, though the mental state that would be required is practically impossible unless you can feel content with life, or at least sustain a non-fear of death.
Well, okay, but you're just speculating at this point. How many billions of people in the world, yet we don't have any quantifiable evidence of this, do we? I mean, really, run some tests by having people in a room trying to kill themselves by thinking... it just doesn't happen.
And typically, as you say, someone who wants to die will actually just do it, not sit around trying to command themselves to die.
And older people have a host of other possible factors that could contribute to their demise, so there is no firm basis for you to state that this is true.
Think placebo/nocebo here, for it to work, you have to subconsciously believe in it. Freud was right about one thing though, the Ego is afraid of dying, and will go to great lengths to prevent it's own non-existence, even if you are suicidal.
Yes. Toss anyone in the ocean and/or drag them down, and literally almost everyone will suddenly fight to live, even if a moment before they were miserable and wanted to die. Seeing death approaching makes almost everyone a wannabe survivor; it's an inborn instinct, to fight for that last breath.