I have to say it is unlikely there will be sanctioned euthanasia for age related decline or dementia. The practicalities of implementing such a system are probably prohibitive, the primary problem being ending the life of someone without the capacity to tell you that is what they want. This is in some ways insurmountable, because although your 50 year old fit and happy self would definitely want to be euthanised if they had severe dementia, how do we 'know' that this same person who now has severe dementia also wants to die?
It is similarly unlikely there will be a test which can predict imminent dementia well enough that you would be happy to euthanise yourself while you still have decision making capacity.
As others have said, what you can do is have some legal document which explains your wishes if you lose capacity to make decisions, where you specify the avoidance of life prolonging therapies, intensive care, resuscitation etc. The other thing which is important is having someone that will take over medical decision making whom you trust will act according to your wishes, in the event you lose decision making capacity. Another aspect of this is making sure friends and family are aware of your wishes. This is a kindness to them as much as anything, so that when the time comes to withhold or withdraw medical treatment, they don't have to ruminate on whether it is the right thing to do.
There are a lot of people with their intellectual capacities intact that express a wish to die. They don't get to die either.
For the most part society doesn't care about your wishes.
I think what it really comes down to incentives. As a society we don't want bullying people into a suicide to be a viable strategy for getting the inheritance money early. Or for getting rid of people in general.
The inheritance money argument would be a good one if it weren't for the fact that care homes and hospitals get the inheritance money instead.
There's still some sentience in middle stage dementia, but by the late stage there's literally no one home any more. It's really not obvious what - never mind who - is being kept alive.
It's certainly not a simple problem. But there are existing systems for legal oversight of care, and of power of attorney over family wealth. It doesn't seem an impossible stretch to extend those to allow early termination in cases of extreme suffering or total and irreversible loss of cognitive function.
We have a neighbour in the very late stages of terminal cancer. He's completely lucid. He wants to go, his family want him to go - this has dragged on for over a year now - and it's hard to see a good moral reason for extending everyone's suffering even further.
There is that, but I still think even in a system where everyone acts in good faith, it is by no means trivial to implement euthanasia for 'diminished states of existence'.
Agreed on incentives. There is no resolvable solution for those incentives. Especially since suicide is easy to plan and not necessarily painful. For example iirc, Robin Williams did it.
Well living is the default, so the burden of proof lies with changing that state.
Alternatively you could argue that they still get out of bed and eat and engage, and perhaps these are signs enough of some desire to live. Anyway, I think there are some philosophical complications which require exploration.
It is similarly unlikely there will be a test which can predict imminent dementia well enough that you would be happy to euthanise yourself while you still have decision making capacity.
As others have said, what you can do is have some legal document which explains your wishes if you lose capacity to make decisions, where you specify the avoidance of life prolonging therapies, intensive care, resuscitation etc. The other thing which is important is having someone that will take over medical decision making whom you trust will act according to your wishes, in the event you lose decision making capacity. Another aspect of this is making sure friends and family are aware of your wishes. This is a kindness to them as much as anything, so that when the time comes to withhold or withdraw medical treatment, they don't have to ruminate on whether it is the right thing to do.