As Jeeves in the P. G. Wodehouse stories might say: Rem acu tetigisti – you’ve put your finger on the sharp point.
The paradigm case for discussions in libertarian theory involves relations between competent adults. The applicability of libertarian principles to incompetents (I use this term non-pejoratively to cover the categories you mention in your query) is contested.
In the course I taught for several years on ‘Anarchy, Law and the State’, which used Rothbard’s The Ethics of Liberty as a core text, I always left his chapter on children until the very end, so controversial did it always turn out to be.
I am not aware of any completely satisfactory treatment of the issues you raise. There may be such a treatment but, if there is, someone needs to point it out to me. I would be just as glad to know of it as you would and I’d be very happy that someone else had done the heavy lifting.
Here is a tentative attempt at answering your questions, revisable in the light of objections, counter-examples and further discussion (which I invite)
Let me take the case of a very young child as a point of discussion (which can then be applied, if indeed it can, to the other categories of incompetents you mention.)
It is hard to understand what would be meant by claiming that a one-year old child is an actual self-owner. Such an entity might be a moral patient (that is, be such that certain things cannot be done to it) but scarcely a moral agent (that is, an entity that can be help morally accountable for its actions). If such a child doesn’t own itself, is it owned by another? Perhaps, but not in the way in which, say, a pencil can be owned. Rather, it has to be owned as something which has the capacity to become a self-owner in time, so that the ownership by another of it is to be exercised for its benefit, a kind of trustee ownership, if you will.
Something like this already applies in our legal system. If a very young child becomes the heir of its deceased parents, the law will not recognise it as having the capacity to exercise actual ownership rights over the property that has been bequeathed to it. Instead, the law will appoint a trustee (if one has not already been appointed) who will be the legal owner of the child’s property and who will be expected to exercise control over that property for the benefit of the child (the beneficial owner) and, eventually, upon the child’s reaching its legal majority, to relinquish full control of its property to the child.
This notion of trustee ownership could be applied not only to a child’s external property but to the child itself, so that its parents (or guardians) are its legal owners who must exercise that ownership for the child’s benefit and ultimately relinquish it to the child.
Now we come to your two questions: (1) who decides?, and (rephrasing your question slightly)(2) who gets to decide the circumstances in which X is not in a position to be an actual self-owner?
Who decides? Trustees, which would be, in the case of incompetent children, its parents or guardians; in the case of incompetent adults, those whom that adult has appointed when competent or, in the case of non-appointment, that adult’s nearest relative or someone appointed by the relevant legal authorities.
Who gets to decide the circumstances…..? Sometimes no serious decision is required. A one-year old child is not competent; a sixteen year old child is normally competent. The problem arises in boundary situations. Here, there is no way, it seems to me, of deciding a priori. Competence is a capacity which should be empirically demonstrable. If a person claims to be competent and can demonstrate it, then that answers the question and no one can override that person’s decisions. If, by any reasonable standard, a person (a young child or an adult who has become senile or an adult with mental problems of a sufficiently serious nature, etc.) cannot demonstrate competence,then, it seems to me, that has to be taken to show their incompetence and others will have to make the relevant decisions on their behals.
There are further issues. Who, if anyone, has an obligation to exercise the role of trustee in cases of incompetence? Can a trusteeship be imposed unilaterally on an unwilling agent?
I have answers to these questions (based on the principles outlines above) but not everyone may agree with me.
In any event, take what I’ve written as a first try, and let me know what you think.