The problem is that there is no way to write a law with the intent of preventing abuse of people [who] are too young to be able to [in?]form themselves to make reasoned decisions about sex or reproduction other than arbitrarily assigning a numerical age as the borderline.
That is demonstrably incorrect. Like any matter of competence, an adequate determination of the actual situation can be ascertained by testing. The relevant issues here would include the various facts of the matter (contraception methods and effectiveness, disease varieties, recognition, and prevention, the technical details of becoming pregnant, etc.); the potential consequences as related to catching and spreading STDs, pregnancy, child-rearing, adoption, abortion, social issues such as reputation, etc., basic statistics on relationship durations and other related matters. Any moderately competent educator could set up an adequate testing regime -- there are no technical barriers to this at all.
What you have is described is what should be the result of a good sex education class. I wish everyone could have that experience. Unfortunately, the social forces you mention later in your reply prevent sex education from being taught properly in many places.
But a test to determine if a person is able to make reasoned decisions about sex and reproduction?
What you have described is, before anything else, a literacy test.
It would be great if we could implement this, but what you have described is eugenics. There is no way to write a law in the USA that even remotely smells of eugenics, and that's a fact.
Hmm, it occurs to me now that what I said is unclear. By "write a law", I mean the entire process from drafting it to getting the legislature to pass it and a governor to sign it.
Also, regarding "And no, agency does not instantly arise like some magic fucking flower when the human body crosses a 16-, 17-, 18-, 19-, 20-, or 21-year old "finish line".
This is a straw man. No one makes the claim as you stated it. We know some people under 18 years are competent to sign contracts, and we know that some people will never be competent.
The idea is that we guess when most people at that age will be competent to make reasoned decisions, as well as the societal benefits to allow and/or harm to prevent for the behavior under consideration, whether driving, signing contracts, drinking, or having consensual sex.
Another problem is that setting a numerical age as the borderline does not prevent abuse of the incompetent, nor does it ensure the individual can make reasoned decisions. All it does is create a legal nightmare for young people and anyone who might be involved with them.
That is why in addition to laws protecting the young from exploitation, there are laws protecting the mentally retarded and insane. Admittedly, this is one area that is a legal nightmare with many contradictory rulings. The problem being that laws preventing exploitation of the mentally handicapped also denies their having sex life in their entire life. I think that courts generally believe each case is unique.
The current state of affairs is toxic, unjustifiable, and guaranteed - known - to cause harm on both sides of the lines drawn. Young people below the line who are competent, and anyone involved with them, are subject to incredibly brutal punishments, de facto extreme compromise of their working, interpersonal, childbearing and residential future(s), gross public shaming, vigilantism and more.
That's why we have the so-called Romeo and Juliet laws that allow sex between people of similar ages.
Unfortunately, many states have not written such laws. This is, as you pointed out, a travesty.
Then you continue with what is basically a string of insults that hides the meaning of whatever you were trying to say.