Sending a teenager to jail for breaking a law is fair because it teaches consequences for illegal actions. Debating wether or not a law should or should not exist aside, abiding by set laws is a test of morality and respect for authority which is essential for comfortable living in society. It is important to teach teenagers to respect authority early on because they will inevitably become the future adults. Programs like "Beyond Scared Straight" are good examples of doing this well. Jail time is much like spanking a child for wrong doing.
On the other hand, when you spank a child you should only do it for the big stuff, not the small things that a time out is more appropriate for. Jail time is a scary thing and it will instill fear in the majority of teenagers regardless of how rebellious they are. The issue is though if you continuous throw the term around or expose them to it often they will become desensitized. Jailing a teenager for just anything in order to instill a respect for all laws regardless of value will eventually make them see authority negatively. So I believe that some problems can be remediated through alternative means to maintain some sort of balance in a child's developing mind.
On average people who spend more than 1 year behind bars will repeat their offence or commit another offence. Prison causes mental illness, seclusion, and worst, while incarcerated, the youth will learn violence as the only means of survival.
Prison is the antithesis of rehabilitation. For a better society we should do more to teach people not to destroy them.
I think that sending people who commit crimes to prison when young has positive and negative effects. The positive mainly being that it instills a fear of going to jail, therefore becoming a deterrent like mentioned previously. But it also robs these young people of a possibly rewarding life not only for the individual but for the people that may benefit from this person's research or work throughout their lifetime.
Criminals by definition are those who commit crimes. Whether or not a teenager goes to jail or not does not change the fact that he has committed a crime, and is a criminal. The idea of "jail" exists to deter teenagers from committing crimes, and is a form of punishment. Those who fear the pain of going to jail more than the pleasure of committing the crime will not commit the crime, and therefore will not become criminals. So Jail actually reduces the amount of "criminals."