In the end I think the person themselves probably knows
My dad is early 70s and in perfect health. He wants to go on living. He isn't depressed or wanting to die but equally he lost the love of his life when my mum died, he has seen his children grow up and have children of their own and he watched both his parents die a slow death from dementia.
If he could choose to die now, which to me of course is too early, or in 10 or 15 years with dementia in a care home I absolutely know he would choose to go 'early'. He has a morbid fear of dementia and care homes after his experiences with my grandparents and I would not wish for him to face that.
Unselfishly I hope he gets his wish even if my selfish instinct is to keep him around as long as I can.