Dershowitz had a reasonable point in his essay about age of consent, and that is that America's many state laws are insane.
I think that the concept of "emancipation" should figure in as a factor. Emancipation of a person of minor age effectively makes them an adult for the purpose of agreeing to consent, making purchases, taking responsibility for their actions, etc.
That is a difficult route to follow because in cases where a minor may leave home and act in a responsible and self-sustaining manner for a number of months, if they leave without an explicit agreement, a child might be declared a runaway, or a parent may still be charged with neglect or otherwise be held responsible for consequences of the actions of their minor child.
If the minor is still living at home and is still dependent upon their parents, they're hardly what can be called "emancipated". It seems to boil down to what the courts call "best interests".