Whether a person will be charged as a juvenile or an adult depends both on the crime with which they are charged as well as their age at the time the offense was allegedly committed. For misdemeanor offenses, people will be charged as juveniles if they are 17 years old or younger.
People who are 16 years old or younger and charged with a felony offense will generally be tried in juvenile court. However, prosecutors may use their discretion in certain cases to try a felony juvenile offender as an adult. If the prosecution elects to do so, the juvenile's case will instead be heard in adult court and he or she will face adult sentencing.
When a juvenile is arrested, he or she may be released to his or her parents or guardian or they may be held in a secure facility. The decision whether or not to hold them in a secure facility must be made within 12 hours for juveniles between the ages of 12 and 16 or within 24 hours if the offense alleged is a violent one. Whether a juvenile is tried as an adult or not, he or she must be held separately from adults with at least a sight and sound barrier if he or she is being held in a county jail.
If a prosecutor seeks and obtains certification as an adult for a juvenile, the juvenile will face the possibility of incarceration in adult prison if he or she is subsequently convicted regardless of age. It is often very important for juveniles and their families to seek the help of a criminal defense attorney who may be able to successfully argue against moving the case to a criminal courthouse.
Source: Illinois Legal Aid, "Juveniles in the Criminal Justice System", accessed on Feb. 3, 2015