Unfortunately you have just described the human condition, we all are ageing and die and don't know when it will happen. What if you have a chronic pain condition but could have an accident tomorrow or live for years it's no difference.

If you don't know when then how is that different to a person with a chronic condition?

I believe in the right to choose, but its not simple.

You could argue choosing to die could be depression, in which case mentally ill. Does that mean you do not have the mental capacity to make that decision because your unwell?
I don't think it is as simple as only thinking in context of the terminally ill either. The only condition that I'm resolute the person should be allowed to die is if you were a vegetable, or a mind locked inside your body then you should have the right to choose in case of that happening.

In terms of mentally ill or terminally ill I'm not so clear cut in my opinions. I have tried to kill myself numerous times, if I had the choice I would have taken it as living is too painful. Then I know it's a matter of riding the wave until I'm stable, I'm mentally ill and should not have the power to choose.

Tricky question with no right answer for sure!