I think part of the problem (and there are many) is that life sentences don't always mean life in prison and shorter sentences don't always mean what they say either. When you can sentence someone to 40 years and they get out in 10, if you really want them in for 40 years, you almost have to sentence them for life. I'm not saying this is exactly how it works, but some of this is almost certainly at play.

I have no problem with life sentences for non-homicidal crimes. If someone rapes 50 people at knife point, they should go to jail for life. The possibility of rehabilitation is irrelevant at that point. The right to liberty can be forfeited just like any other right. If you commit certain crimes, you can lose your right to vote or to bear arms. I think this extends to the right to liberty as well.

The whole issue of rehabilitation is up for debate. That a murderer won't do it again doesn't undo what they did. If someone steals $1000 from you, you're still out $1000 whether it's the last time they steal or not. I think the main goal of punishment, where applicable, should be restitution. If you steal someone's car, you should be forced to pay them back for it, and then punitive damages should be assessed on top of that as a deterrent from future crime, because if you steal 1000 cars and get caught for only 999 of them, you shouldn't come out ahead.

In crimes where restitution is not possible (rape and murder are the obvious examples), outright punishment (for no one's benefit except future potential victims) is warranted in my mind. The law can only reasonably be maintained with a threat of negative consequences which fit the crime. A $100 fine would not be a reasonable deterrent for murder because it doesn't fit the crime. Murder rates would certainly rise if that were the punishment. A threat of course is meaningless if not followed though on, so the punishment for crimes which cannot be repaid must be harsh enough to serve as a deterrent and enforced regularly enough to continue to deter. Life in prison does not constitute as strong a deterrent if the criminal knows it won't be enforced. The death penalty in every state but Texas probably constitutes no additional deterrent at all given how rarely it's enforced.

To me, non-enforced laws are a serious problem. They send the message that we're not really serious about this whole law thing. This is why people get so upset when they get speeding tickets, because the law is presented as a joke. There is no serious attempt to prevent speeding, yet you can still get a ticket for it. This is a problem.

It is vital that laws for more serious crimes are not presented in as flippant a manner.

The question is basically what constitutes a punishment which sufficiently fits the crime to serve as a deterrent. If a life sentence is necessary to meet this condition for murder by adults, what changes with teenagers?

EDIT: I want to add an important distinction here. There are two separate things I am saying here. A punishment should fit the crime, and a punishment should serve as a deterrent. The punishment is just because it fits the crime. It is executed at all because it is a deterrent. The fitting of punishment to crime is the justification of punishment. The deterrence is the reason for punishment. These are separate issues entirely. If it was impossible to change crime rates regardless how you punished crime, there would be no reason to punish crime at all, even if it was just to do so. Similarly if killing people for theft prevented all theft (thus serving as a perfect deterrent), it wouldn't be just simply because there was a reason to do it.

I think the correct way to assess how children and adults (older teenagers being counted as adults) should be sentenced differently is by considering the difference in the crime, not the difference in the person. If a six-year old kid shoots and kills someone, their crime is not the same as if a 36-year old does it. The intent is part of the crime, and a six-year old's intent cannot possibly be the same as that of a 36-year old, simply due to their lack of understand of guns and life and death.

That said, I don't think the point of sentencing children differently is that children can be rehabilitated.

Rehabilitation only addresses the issue of prevention of further crime by the same person, which is not the entire issue. Part of the issue is maintaining the force of the law as a deterrent to all future potential criminals. But you shouldn't read this as saying that a criminal is paying not for his own crimes but for those of others. The "payment" for murder cannot be made by anyone.

There are many reasons not to use the death penalty. Enough to convince me that it should only be used in cases of especially heinous or repeat offenses. Nevertheless, I think that if you intentionally kill someone, your right to life is essentially forfeited. Life imprisonment is a concession to the vast number of imperfections in the death penalty (not the least of which is the possibility of sentencing innocent people), not a statement that the criminal deserves to continue living.

Also please understand that I am talking about deliberate, premeditated murder here. If someone in a drunken bar fight intends to punch someone in the face and accidentally kills them, they should be punished differently, not because we expect they won't do it again, but because their crime is fundamentally different. (Of course this brings up an entirely separate issue. If being in a drunken bar fight means you might end up killing someone, then being in a drunken bar fight constitutes endangerment to the lives of the people you are fighting, which is a completely separate crime from the actual fight or the results thereof. There is, however, very little punishment for endangerment in the U.S., which is understandable given how difficult it is to pin down.)