>Would anyone provide a counterargument?

Sure. Do you mind if I'm a social nihilist?

Once upon a time, there was a place in mild prosperity, where there was a reasonable amount of wealth and almost any person who worked hard enough and didn't run into any terrible luck could eke out a living. Near this land there lived a fully sentient and conscious, territorial dragon who hid in a lair guarding innumerable treasures. The people largely respected the dragon's right to its property, and when foolhardy wealth-seekers intruded on its lair, people largely allowed the dragon to go about its business, grievously injuring those intruders. But when a child, or an insane person, or a wanderer who didn't know any better strayed into the dragon's lair, people pooled their efforts and sent envoys to rescue them before it was too late. Of course, nobody was terribly concerned that this process made the dragon any less threatening, or that people would try to intrude on the dragon expecting to be saved.

You may by now have realized that the dragon is a stand-in for our criminal justice system, and the criteria are the age of majority, the insanity defense, and mens rea. You see, it is a common lament of libertarians that the criminal justice system exists to control people, which, to me, is only slightly inaccurate. The criminal justice system exists because we can't control people, not even to the extent of "don't kill each other".

We created a giant, bloodthirsty monster that likes to kill people who step out of line, and we called it "The System". While we're not sure precisely how The System prevents crime, we do know that societies with a System experience less crime than those who don't. We haven't, for the most part, done enough research to determine what particular characteristics are good for preventing crime, and to a large extent, of the research we have done, we ignore the findings. We have the System because it's been around for a very long time and we're all -- quite rightly -- terrified of what our society would look like without it.

Among the people who work with The System, there is little illusion that the whole thing was designed with the efficiency and elegance of closed-source software commissioned by business majors. But it's not terrifically important if we steal off some of the most sympathetic of its prey, because, on balance, the System continues to exist as a viable threat to any would-be offenders.

So we don't give 12-year-olds a free pass because we think they aren't aware of their actions. We do it because we know that a 12-year-old is at an extraordinarily vulnerable time in their life and oftentimes a pretrial diversion program can mean the difference between a postgraduate degree and a life on the streets. We do it because we understand that we don't sufficiently undermine the System by allowing 12-year-olds to get off, because for the most part kids that age aren't going to try to exploit our kindness. And of course we do it because there was never anything ethical about the System in the first place, merely necessary, and most of us can all too easily imagine how artificial and distant some of the greater structure of society seemed to us as children. Most people can't stomach the idea of sending a kid to jail.

Law isn't based on reason, evidence, or morality. It's based on fear. Specifically, fear that society wouldn't work without it. But we aren't scared of 12-year-olds.