No - not missing it. Just don't think it's a real distinction.
The ancient common law put criminal responsibility (knowing right from wrong) at about age 7. That is STILL the case - the purpose of the juvenile system is not to diminish responsibility but twofold: to rehabilitate if possible, and to keep juveniles out of the prison system so that they are not victimized.
Actually it didn't. Common law (the doctrine of doli incapax to be precise) assumed a child was not criminally responsible in most cases until the age of 14 - which at the time was fairly consistent with the age he was allowed to carry out most adult responsibilities. There were very rare cases where a child under that age might be found to be criminally liable but they were rare and unusual. Seven was set at the absolute limit for these rare cases - it was never the 'normal' limit under common law.
Common law can be superseded by both courts and legislatures, so it's not particularly relevant to modern discussions, but it was what it was and it said what it said.
And if you're thinking that kids are like puppies, you are denying both their intelligence and their humanity.
No, I don't think kids are like puppies - and I'm frankly amazed that anybody might think I do. The point of the quote I gave isn't whether children are like puppies or not. I'll single out the bits I consider most important just to make it clear.
"Never mind. Long enough. It means that such punishment is so unusual as to be significant, to deter, to instruct. Back to these young criminals. They probably were not spanked as babies; they certainly were not flogged for their crimes. The usual sequence was: for a first offense, a warning a scolding, often without trial. After several offenses a sentence of confinement but with sentence suspended and the youngster placed on probation. A boy might be arrested many times and convicted several times before he was punished and then it would be merely confinement, with others like him from whom he learned still more criminal habits. If he kept out of major trouble while confined, he could usually evade most of even that mild punishment, be given probation 'paroled' in the jargon of the times.
"This incredible sequence could go on for years while his crimes increased in frequency and viciousness, with no punishment whatever save rare dull-but-comfortable confinements. Then suddenly, usually by law on his eighteenth birthday, this so-called 'juvenile delinquent' becomes an adult criminal and sometimes wound up in only weeks or months in a death cell awaiting execution for murder.
It's actually a separate point to the other one, which is why I put it into a separate post. There are two issues here that are relevant as far as I am concerned. The first is whether or not it is sensible to treat people as irresponsible children for some purposes and as responsible adults for others. I don't believe that is sensible or reasonable.
The second issue - the one I quoted from Starship Troopers about - relates to the fact that our society is creating more and more delinquents because we're not punishing children in a way that's appropriate for children. We let them get away with so many things (or nearly so) until they suddenly go too far for us to ignore it. And then we tend to come down on them like a pile of bricks. Our society isn't doing what it needs to do to stop these children becoming serious criminals. Once they become such, we have to treat them as such - but we should be doing a better job of preventing them reaching that stage.
I'm a teacher - my students don't turn into juvenile delinquents? Why - because their parents discipline them, and so do their teachers (with the approval of the parents in our case). Every day I see kids who aren't being disciplined and a lot of them do wind up as criminals. It's their choices, but it's not just their choice.
I lost my parents when I was nine. I was lucky. Other people took on the responsibility of seeing I was brought up right. There's too many kids now who nobody is doing that for.
And of course Heinlein is just getting everybody's juices flowing with his argument (and you went for it). He starts with a completely false premise: that the juvenile system is 'just a slap on the wrist'. People love to claim that, and of course the writer, to achieve his desired effect, knows that. But he is seizing on a popular (and erroneous) idea for his own purposes.
Certainly there are folks who fall through the cracks, juvenile officers who are not vigilant, juvenile judges who are overly lenient, but a quick read of the appeals reports from any state intermediate court will give the lie to the idea that juvenile offenders aren't punished. That system includes a complete range of punishments up to and including imprisonment, and believe me they are imposed.
So your first premise is inaccurate.
As to the question of criminal responsibility, it is true that I was painting with a broad brush and speaking rather generally. You're correct that at common law 7-14 was a fuzzy area. But the point is not that that law is still in force, or even the details of it, but that it has always been recognized that a child may have criminal culpability long before he has the ability to enter into a contract or vote. And the younger end of the scale depends, just at it depends in the modern system, on multiple factors. That's why it was, as you note, a rare case for a child as young as 7 to have full responsibility for a crime imposed upon him. It's rare now, as well.
The other question is one of predictability, and balancing the situation of the juvenile against the danger to the public. Is a child who can commit rape and murder at age 14 capable of being rehabilitated? And are you willing to sit on a parole board that releases that individual, only to have him reoffend?