Why might that be problematic? In his article "The Contradictions of Juvenile Crime and Punishment," found in the Summer 2010 edition of Daedalus, Jeffrey Fagan offers several reasons. First, from a rehabilitative standpoint, it backfires:
... even short-term exposure for youths to adult prisons has risks for youths and for public safety. To the extent that legislators ignored these risks, the wholesale transfer of minors to the criminal courts was a reckless experiment. A robust body of research shows that recidivism rates are in fact higher for youths sentenced as adults, after controlling for relevant offender and offense characteristics....Why is this the case?
One explanation for the elevated recidivism rates may be the effects of adolescents' exposure to prison life and adult convicts. While likely to be separated physically from older inmates, the institutional climate on the youth side may hardly differ from other blocks in the prison: the separation may be one of degree rather than kind. Indeed, it may even worsen the chaos and violence of correctional confinement by concentrating youths who are at their peak ages of criminality and diminished self-control.The experience is more psychically damaging to youths, as well:
Only a few studies have compared the correctional experiences of youths in prisons and juvenile incarceration, but all agree that placing youths in prisons comes at a cost: they are less likely to receive education and other essential services, they are more likely to be victims of physical violence, and they manifest a variety of psychological symptoms.Of course, the affirmative rejoinder is that juveniles who have been charged with violent felonies are potentially beyond rehabilitating in first place--and that the reluctance of society to punish juveniles like adults might give juveniles an inflated sense of invincibility.