You don't get to; at least most of us don't. There's no "off" switch that you can hit when you've had enough. Juliet says "If all else fail, myself have power to die," and she means it. Most of us are more Hamlet than Juliet, and "conscience does make cowards of us all."
Aging is a process of accepting what you can't do and what you will accept as you wait for the end of life. How much does your knee or hip or shoulder have to hurt before you let someone slice it open and replace your parts with steel, titanium, or corpse pieces? How willing are you to shoot up every day with insulin or some other drug? At twenty you might have said, "I'd rather die than have a colostomy," but at sixty, you may need to reconsider.
Research shows that the longer we live, the longer we want to live, but there are times when I wonder where the balance point is. It's heresy in this country to say you don't want to live (and I'm not feeling suicidal, just ruminative).
We're supposed to be grateful those doctors can keep your heart pounding, however weakly and at whatever cost to your dignity, your resources, and your family.
It's good that aging is gradual. If instead you woke up one morning with dimmed vision, poor hearing, aching joints, and contrary inner parts, you'd be so shocked you'd die of heart failure. If you didn't, looking in the mirror would do it. We don't like watching the gradual sags and shifts of age, but an overnight event would be devastating.
So at what point do you say, "I've declined enough, and now I choose to fall"? In America, that's not allowed. We're supposed to cherish our golden years. We're told that they're wonderful, full of new journeys and beautiful horizons.
That's nice, if only my back didn't keep me from enjoying the journey. And if I could see the horizon.