Who cares about a "fully developed" brain, if there is even such a thing? Human beings at any age are largely irresponsible and irrational and most people never move beyond this. Yet I know that the vast majority of 17-year-olds I have encountered do not mail boxes of ashes to Jews, or anything of the sort. That his brain is not totally "developed" absolves nothing. Should a person with an IQ of 100 serve less time than a person with an IQ of 140, because the latter is more "capable of rationality", or whatever other quality you want to ascribe to the mind? At some point, certainly short after the reasonable age of entry into the working world (probably sixteen) people need to be expected to act decently. Sure, I'm in favor of rehabilitation -- punishment is a revenge-instinct and waste of resources -- but we should do ourselves and these people the service of being honest about what they are: criminals.