In most states persons accused of violating the law before they are 18 years of age are under the jurisdiction of the juvenile courts, and persons accused of violating the law when they are 18 years of age or older are under the jurisdiction of the criminal courts. Some states begin prosecuting persons as adults in the criminal court system when they are 17 years of age, and some states begin prosecuting persons as adults in the criminal court system when they are 16 years of age.

But all states have transfer laws that permit or require criminal prosecution of some young offenders—even though they are still of juvenile age, as defined by the state’s laws. Transfer laws vary from state to state, but generally fall within three categories: (1) judicial waiver laws that allow juvenile courts to waive jurisdiction on a case-by-case basis; (2) prosecutorial discretion or concurrent jurisdiction laws that define a class of cases that may be brought in juvenile court or in criminal court—usually at the discretion of the prosecutor; and (3) statutory exclusion laws that grant criminal courts exclusive jurisdiction over certain classes of cases involving juvenile-age offenders.

The laws that govern when a person of juvenile age may be prosecuted as an adult in the criminal court system are generally located in a state’s statutes.

In Massachusetts, the general rule is that individuals who are under the age of 18 at the time of an alleged offense are under the jurisdiction of the juvenile courts, while those who are 18 or older are prosecuted in the criminal courts. However, Massachusetts does have transfer laws that allow for certain juvenile cases to be moved to adult court. These laws include judicial waiver, where a juvenile court judge has the discretion to transfer a case to adult court after considering factors such as the seriousness of the offense and the juvenile's prior record. There is also statutory exclusion for certain serious crimes, such as murder, where the law mandates that juveniles, typically 14 years of age or older, be tried as adults. Additionally, Massachusetts employs a 'youthful offender' statute, which applies to juveniles between the ages of 14 and 17 who commit serious offenses and can be sentenced as adults. Each of these mechanisms reflects the state's approach to balancing the rehabilitative focus of the juvenile justice system with the need to hold young offenders accountable for serious crimes.