Michael — Re-reading your original post, your initial paragraphs all rely on the “legal age” idea — which just seems arbitrary to me. This is a concession — if even only a small one — to the “social contract” idea, in which you are born into a society with a “legal age is 18” rule, whether you like it or not. You are subject to this rule, whether you like it or not, whether you gave your consent or not. This does not seem to square with a libertarian philosophy of rights.
<< But society/state would need a body which decides when someone is mentally ill and is a danger to others. Not themselves. >>
Same here. You are basically conceding that the society/state DOES have some role where they are “legally authorized” to override an individual’s right of self-determination.
In both of these cases, the concept of self-ownership is subservient to the decisions of the society/State. It concedes that ultimately the power that authorizes self-ownership is not actually the individual, but the State, which claims the right to deprive an individual of self-ownership. Once this concession has been made, what is to stop the State from finding other situations where the State can assert the right to deprive an individual of self-ownership?