Final answer:
The severity of the crime primarily determines the system under which a person is tried, with the age of the offender also playing a role in cases involving juveniles.
Step-by-step explanation:
The factor that determines the system under which a person is tried is primarily the severity of the crime. In the U.S. criminal justice system, the level of charge (e.g., misdemeanor versus felony) often determines the pathway through the court system, with more serious charges generally resulting in more formal and greater procedural safeguards.
The age of the offender may also be a determining factor, as juveniles are typically tried in a separate juvenile justice system. Other factors, such as legal representation and public opinion, may influence aspects of the process, but they do not determine the system under which a person is tried.