First of all, we have to be clear on the fact that not everyone would agree that the death penalty should be off the table when it comes to juvenile offenders. There are certainly those who would argue that juveniles that commit crimes that are horrific enough should be put to death just as adults who commit such crimes would be. However, the Supreme Court has ruled that it is not constitutional to execute anyone for a crime that they committed while they were under the age of 18. Let us look at why someone might think in this way.
The major reason why we think in this way is that we do not believe that juveniles are fully capable of making their own decisions. Think about all of the things that people under 18 are not allowed to do. They are not allowed to vote because they are not deemed capable of thinking clearly enough about political and social issues to be allowed to do so. They cannot sign contracts because they are not deemed to be old enough to commit to such things. They cannot use tobacco because they are not believed to be old enough to make their own decisions about how they want to treat their bodies. For the same reason, in many places, they are not allowed to get tattoos without parental permission. They are not allowed to buy lottery tickets for fear that they cannot control their impulse to gamble.
From all of these restrictions that we place on young people under the age of 18, it is very clear that we do not really think that they are capable of making decisions for themselves. If this is the case, how can we then turn around and say that they are capable of fully understanding a decision that they make to kill someone? How can we say that they are too immature to fully understand the implications of getting a tattoo, but that they are mature enough to fully understand the implications of killing someone? This would not make any sense.
Put in more measured terms, the idea is that psychologists would say that teens do not have the maturity to be held fully accountable for their actions in the same way that adults are. Because we believe that teens lack maturity, we are reluctant to kill them as punishment for crimes that they have committed.
There must surely be some line below which a child is too young to be held completely responsible for their crimes. If we say that 18 year-olds cannot do any of the things listed above (as well as many other things they are banned from doing), it would not be right to, at the same time, say that they are responsible enough for their actions that they should be executed.