21st Century Law Enforcement, Crime Prevention, and Victims Assistance Act - Extends: (1) the community policing (COPS) program by providing for hiring more police and prosecutors; (2) Project Exile; and (3) the Youth Crime Gun Interdiction Initiative.

Imposes a gun ban for juveniles convicted or adjudicated delinquent for violent crimes. Requires revocation of a firearms dealer's license for failing to have secure gun safety devices available for sale with firearms. Requires criminal background checks for all gun sales at gun shows.

Establishes new crimes and increases penalties for killing Federal officers. Enhances the penalty for assaults and threats against Federal officials engaged in official duties. Provides a limited protective function privilege for Secret Service agents.

Prohibits the interstate franchising of street gangs. Increases penalties for crimes involving the use of body armor or laser-sighting devices. Doubles the maximum criminal penalties for: (1) using or threatening physical violence against witnesses; and (2) voluntary manslaughter. Provides for jurisdiction over foreign banks engaged in money laundering that have U.S. accounts and for criminal forfeiture for money laundering conspiracies.

Provides: (1) a model Bill of Rights for crime victims in the Federal system; and (2) assistance for shelters and transitional housing for victims of domestic violence. Makes available to States grants for victim-related training and notification systems. Authorizes grants for pilot programs to operate Victim Ombudsman Information Centers.

Establishes new offenses and increases penalties for crimes against senior citizens. Reauthorizes the Juvenile Justice and Delinquency Prevention Act and creates a new juvenile justice block grant program.