Elderly adults often pose, or are perceived to pose, a risk to themselves or others. Others often also have the will, and sometimes the ability, to intervene in their lives: to take them from their homes, force medical treatment on them, and prevent them from spending their money as they see fit. Thus the question “To what extent, and for what purposes, can others interfere with the liberty of the individual?” takes on particular relevance for elderly adults. This is the question John Stuart Mill addressed in his essay On Liberty, and our aim is to adapt his theory of liberty to guide those who must make decisions about the old. We argue that individuals can be interfered with in their own interest and in the interest of others. It is apt to interfere in their own interest if and only if they are putting themselves at significant risk, are encumbered, i.e. are incompetent or subject to some other judgement-distorting condition, and interference will be: (a) effective, (b) not generative of evils greater than those it prevents, (c) the mildest possible to curb the evil, (d) not discriminatory, and (e), unless the interference has utility approaching absolute necessity, thought justified. It is apt to interfere in the interest of others if and only if they are putting others at significant risk and the same five conditions are satisfied. As we hope our adaptation of Mill's theory of liberty will have practical effect, we illustrate it with examples, relate it to the law in British Columbia, and state it in the form of a decision-tree.