I think the natural legal distinction is puberty. And the agents which have been traditionally responsible for policing this - far more effectively, by the way, than the State - were family members. Mom, dad, grandparents, and so on. They are at once the people most likely to have the children’s best interests at heart and the people closest to the children and, thus, most likely to detect and prevent abuse of any kind.
A general suspicion of parents is a hallmark of the statist mentality. And child “protection” agencies should send shivers down the spine of any decent human being. These bureaucracies are merciless grist mills that chew up families (mostly poor families) and spit them out. Many of the children that end up in the foster care system are systematically abused and neglected by the foster parents who have either signed up for the money or for the opportunity to exploit children with absolutely no remaining safety net.
Some pedophiles try to apply libertarian arguments to child sex. “As long as the child is consenting, what’s the problem?” There are several cases here:
a) The parents don’t agree to the child engaging in sex. In this case, it doesn’t matter what the child wants, the parents’ legal right to exclude others from interacting with the child on whatever basis they please is sufficient to stop this. Whether it’s a tort is a different matter (and I think the law in an unhampered market in law would treat it as a tort).
b) The parents agree to the sex but are not themselves doing it. In this case, the extended family should be able to intervene on the basis that abuse is occurring and be able to seek to transfer the parenting rights to themselves.
c) No one is protecting the child (parents or extended family). In this case, I think that even strangers would then have a right to intervene and something along the lines of an “abandoned property” argument could be applied here - the people who naturally have title to the parenting rights simply are not exercising them. Thus, those rights have been abandoned and can be homestead by anyone, such as a charity for orphans or whatever. Once the rights have been transferred, the child will again be protected.
Here is a slightly similar thread.
Clayton -