Actually Sachs, if you are the parents, and the said person is living at home, I think, in that persons best interests, (and your own), you can, make that decision, although, on the legal side, once that person has become an adult, the state becomes his/her guardian.
tgerlily, a good point, but as said, this person is living with his/her parents, so you have to take into account the stress levels involved all round, and don't forget, they can't go to the authorities, because of past experiences.
A Moral dilemma which only the parents can answer, if they come to a decision.
Living wills are fore the mentally able, the person i'm talking about, (a real life person), is twentyfour, with a mental age of approx one year, able bodied, but does not speak or understand the spoken word.
The Day centre have said that soon they can no longer take her/him, the person is too strong now for the mother to take out, no help from family, and both mother and father are at their wits end
I know them personally, and they are constntly worried about the future, but, should they take the same course of action as the parents in the question, they would be legally murderers, but I for one, wouldn't blame them.