This article discusses the dilemma faced by people with dementia and Alzheimer's. - decide while you are able to make this decision, but then it means it's very likely too early to die - wait until your cognitive abilities have declined to the point the more able version of you would have wanted to die, but by then you can't make that decision anymore

It seems like a mostly solved problem, in the Netherlands and Canada at least, where people can make advanced decisions that waive the need for a final agreement at the moment of euthanasia itself.