When does self-ownership start according to libertarians?

Self-ownership starts when you are capable of taking care of yourself. Until then you are the responsibility of the caretaker(s), normally the parents are the caretakers of a child.

This happens at 18 years of age in most countries by law but I would say that my parents gave me full freedom on my 15′th birthday. After that I was pretty much just a tenant with free food and lodging.

The non aggression principle has nothing to do with this! You are not allowed to beat up a child or even kick a dog! Regardless of its age. Violence is only allowed to be used in defence when attacked.

If a woman considers her fetus to be an invader she is allowed to remove it from her body at any time at her own expense.

If she has given birth to a child that child is immediately after birth protected by the non aggression principle and she has the responsibility to care for it.

So there are three events that control this:

Life starts at conception but until birth this life is totally controlled by the pregnant woman.

NAP starts at birth.

Self-ownership starts at legal age or when your parents say so. Usually this is a gradual process where freedom is given in increments as you grow older.

What the legal age ought to be can of course be discussed. Most countries have more than one. I live in Sweden and here 15 and 18 are both used. When you’re 15 you can have consentual sex and be prosecuted for crimes, go to any movie and do some other things but you can’t drive a car, buy cigarrettes or vote until you’re 18 and you have to be 20 to buy booze. Self-ownership is a gradual process…

It starts when you can and are willing to take responsibility for your own actions.

In most cases, this is a gradual process. We can point to the beginning at around puberty and the graduation when you can take on a full set of responsibility like studying at college or supporting yourself with a job.

Of course, there are people who will show that they actually can't take responsibility for various reasons. They will have some of their rights taken away, such as being sent to prison or a nursing home.

I assume different libertarians have different views. In my case, I think you can't assert a right to self ownership (or any right) until you are self aware. It then becomes a philosophical question as to when non-self aware beings should have their rights protected by others.

Moral agency commences at birth. Then you are autonomous— detached from another moral agent. At that point you commence gathering the material info for framing moral imperatives. I.e. perception or identification leading to classification of experience.