First of all, please be aware that every state in the United States has different laws that have to do with when a juvenile offender may be tried as an adult. The link below has various tables that give the details of each state’s laws. Please consult that link for details about your state.
In general, juveniles may be tried in adult courts for any one of three reasons, or for a combination of those reasons. The reasons are: the gravity of the crime with which they are charged, their age at the time of the alleged crime, and other factors such as the likelihood that they can be rehabilitated.
Juveniles who are charged with more serious crimes are much more likely to be tried as adults. We can see this, for example, in the case of the Wisconsin girls who are being tried as adults for an attempted murder they allegedly committed when they were 12. The idea here is that some crimes are so heinous that their perpetrators need to be punished more severely than can be done in the juvenile system. In some states, it is mandatory to try juveniles as adults for the most serious offenses.
Juveniles are much more likely to be tried as adults if they are relatively old. Many of the states that have laws permitting juveniles to be tried as adults set age limits below which juveniles cannot be tried as adults. One reason for having a separate juvenile justice system is the idea that juveniles are less mature and are therefore less responsible for their actions. As juveniles get closer to adulthood, states tend to think that they are more responsible and can therefore be tried as adults.
Finally, juveniles are more likely to be tried as adults if other factors suggest that they should be. One of the reasons for having a separate juvenile justice system is the idea that it is more possible to rehabilitate juveniles than adults. When there are factors in a juvenile’s record that suggest that he or she cannot be rehabilitated, courts are more likely to allow him or her to be tried as an adult.
Thus, juveniles are more likely to be tried as adults when they allegedly commit more serious crimes, when they are relatively old, and/or when they appear to be less likely to be rehabilitated through the juvenile system.