S.W.
There is a thing called independence. They may not know it but they are fighting it with all they have to keep it.
It is hard to be on the sidelines and watch the decline happen. But there is not too much you can do until they admit to getting older and needing help. If you try, they will resent you for the fact that they think you think they are "old" and not capable of doing things for themselves.
Being it has been just the two of them all these years it will be hard on the one left behind. In fact the one left behind may grieve to the point that they also go. They usually don't last too long alone.
I haven't gotten to this point but I am sure one day I will be there. When that day comes, it will be hard to know that you can't do that anymore and you need outside help with things in your life.
Good luck to you. Get a few books from the library about aging and what you may be able to do to help them.
the other S.
PS My husband has gone through some difficult medical issues and he has the thought process of an 80 year old some days even though he is in his mid 60s. He also realizes that there are changes and it frustrates him that he is slower now and not able to express his thoughts as he once could. But it is was it is a part of the life cycle.