In the beginning of the book "love in the time of cholera" a man has committed suicide because he would rather die than suffer the weakness forgetfulness and discomfort of old age ! what would you do ? i Personally would rather die than be weak one day…
I wouldn't like my loved ones to see me suffering. I would want to die before I started being unable to support myself and talk properly, before the second childhood.
If I was old and suffering, forgetful and very weak my answer would be yes.
Only if I was fit and well would I want to continue with life.
I would rather suffer. Who wants to die???? There are still many things you can do without being strong and having a perfect memory. 
I am happy to suffer in old age, to a point. but I have a seen family member with demenita where they cannot feed/dress/wash themselves, don't know who they are or who anyone else is and are cared for in homes by people who don't actually care at all. so there will be a cut off point.
Well, I am a scrap off the 80 mark, but as long as I can maintain my independence I am OK. less energy than I used to have, tire more quickly, can't do all the things I used to do and have less resilience – ie life's knocks knock harder and it takes longer to get them into perspective.
Three dear friends have died fighting diseases and two of us have seen offspring die (at 34 and 51 years of age).
I don't feel wise, but I can now prioritise – the important and non-important,
The world is still a place full of wonder and I consider myself lucky in spite of life's knocks. Love to see the children going to and from the school at the end of the street, envy them their energy. Love the changing seasons. Love the freedom from the stresses of youth.
So no, the weakness is starting, eyesight not that good, but I'm not inclined to opt out yet!
What I would do if in great pain or if I was becoming physically incapable I do not know. Would I find the resources to cope? I don't know.
Have thought of having "do not Resuscitate" tattooed across my chest (!) and have put a note in with my patient records to say no heroic treatment, just keep me comfortable.