Yes, I believe that juvenile offenders under the age of 17 should be penalized for their actions. If a juvenile commits a crime they should face the consequences of the crime regardless of their age. There are so many juvenile offenses because juveniles understand that nothing will happen to them for their actions. I believe if you commit an adult crime then you should expect an adult punishment. This would deter the actions of others as well.
There should only be certain penalties for juvenile offenders, such as community service. It is important that they are not imprisoned or isolated from society at large, and that their education be allowed to continue, as the only thing that will happen in these cases will be the perpetuation of an ugly cycle.
I think that from 10 and up you are pretty aware of the consequences to your actions. You might not be fully mature, but you should still be penalized for your actions. You should be punished for the severity of your crime, not the age of your body when committing it.
Regardless of a person's age, s/he should be held responsible for committing a crime. If a crime has no repercussions, then what is to keep the person from attempting it again? It's better to try to curb criminal behavior early on than to just condone it and figure the person will change over time. The punishment may be different and less severe than the punishment an adult receives, such as juvenile hall or being forced to stay at home, but they should still be punished.
Anyone who does wrong and harms someone else should be held accountable for it, no matter what their age, especially if it was intended. However, for a young person the punishment should include some aspect of education and assume that there is rehabilitation possible. They should usually not be housed with adult offenders.
I believe that juveniles at any age with the intention to commit a serious crime and actually end up committing a serious crime should suffer the consequences of their crime. If a 15 year plans to kill someone and actually kills someone, why shouldn't they get penalized. They made the conscious choice to take the life of someone.
Juveniles need to be punished both as a deterrent and as punishment just as adults do. Obviously there need to be some special considerations taken for juveniles but in many cases a person under the age of 17 is already on a path of organized self destruction and fully aware of the consequences of their action as well as the morality of them. Even now in cases of "kiddie" gangs they will have the youngest members commit deeds knowing that their age will absolve or mitigate the punishment.