In the U.S. the overwhelming majority of agencies use twenty one years of age as the hiring standard. There are some (very few) that hire younger than that.
From your tax dollars!
Police etc
The majority of law enforcement agencies in America are operated by local governments, such as city police departments or county sheriffs' offices. There are also state police agencies that have jurisdiction within their respective states. Additionally, there are federal law enforcement agencies like the FBI and DEA.
The three agencies in criminal justice (in a consensus model, like in the United States) are the police, criminal courts, and correctional agencies. A case will go through the agencies in that order, starting with the police (which identifies crimes) and ending with the correctional agencies (once the agency reforms and reintroduces the convict to the public). The police identify and investigate crimes, the courts hold trials and sentence those found guilty of crimes, and the correctional agencies carry out the sentences that the courts set.
I believe you need to refer to Dr. Krimmel's study which it identifies several consolidation within police agencies.
There are a multitude of agencies that fight crime in the US. They range from local police departments to county sheriffs to state police to national agencies like the FBI.
Police agencies are organized differently, usually depending on number of employees. Starting from the bottom/up there are: Patrol Officers Sergeants (who supervise patrol officers) Lieutenants (who supervise Sergeants) Captains Deputy Chief(s) Chief Police agencies are classified as paramilitary which means it is structured similar to the Army, Marines etc.
Law Enforcement Agencies is the big name for Police, CIA, FBI, SWAT ect.
There are approximately 18,000 law enforcement agencies in the United States, ranging from local municipal police departments to federal agencies like the FBI and DEA.
73
The police can and will investigate internal wrongdoing themselves - most agencies have an Internal Affairs Division. Also the Prosecutor's Office, or the Office of the Attorney General of the state.