Arrests For Crimes
A charge must be identified very early in the criminal justice process, typically upon booking an arrested person at a police station or jail, which will usually take place minutes to hours after an arrest is made.
At the latest, a provisional charge will be revealed at a first appearance before a judge (which will usually take place within a day, or sometimes a bit longer on a weekend or holiday in rural areas whether it should or not), although the charges identified at that time can be amended later in the process. A plea of guilty or not guilty is entered at an arraignment, which sometimes is combined with a first court appearance and sometimes is conducted separately.
A preliminary hearing happens much later in the criminal justice process, after an arraignment (which is sometimes combined with a first court appearance), and after counsel is assigned to represent the person who is charged. A preliminary hearing is an adversarial mini-trial before a judge at which the prosecution presents evidence sufficient to establish probable cause on previously disclosed charged in a somewhat summary fashion (not necessarily everything to establish guilt beyond a reasonable doubt), and the attorney for the defendant can cross-examine witnesses, object to questions improper under the rules of evidence, and introduce exonerating evidence (although this last option is only rarely utilized). A preliminary hearing is a substitute for a grand jury indictment as a means to screen whether charges are supported by probable cause.
Usually, but not in absolutely all cases, if the arrest is pursuant to an arrest warrant, the arrest warrant will be presented to the person being arrested immediately prior to, at the time of the arrest, or immediately after the arrest is made, and the arrest warrant will provide some insight into the charge.
But, unless a state statute or court rule provides otherwise, a charge does not usually need to be disclosed during the actual process of making an arrest, or while the person arrested is being transported to a police station or jail for booking, although the charge often is disclosed at that time or even before the arrest is made.
Frequently, someone is read their Miranda rights before the charge is identified.
Some of the fine details are a matter of state specific court rules and statutes, but the general outline of the process described above is the U.S. constitutional requirement.
The process for arresting a minor is not very different from the process for arresting an adult, but usually, when a minor is arrested, a parent or guardian must be promptly notified of the arrest and the charge, if possible.
Many states have a minimum age for arresting a minor for committing a crime or juvenile delinquency offense, which is typically somewhere in the range from age seven to age twelve, although there is considerable variation on this point. This is an area of the law where active efforts to pass reform legislation to take a less punitive approach with younger minors are underway.
Minors Taken Into Custody For Other Reasons
Also, while adults are overwhelmingly arrested on suspicion of having committed a crime, a minor is sometime placed under law enforcement or social services custody in a manner that looks like an arrest, but isn't, for a reason other than a charge that the minor is actually believed to have committed a crime.
For example, suppose that a five year old child is found in an apartment at which two adults presumed to be the child's mother and father, are found dead (the reason for their deaths doesn't really matter). In that situation, the law enforcement officer at the scene would generally take physical custody of the child pending the arrival of a social services officer at the scene who would then take physical custody of the child from the law enforcement officer until another suitable guardian could be located, even though there is no suspicion that the child committed a crime.
The name for this process varies from jurisdiction to jurisdiction.
In New York State, for example, this is called "person in need of supervision" case (PINS for short), which is sometimes thought of as a civil custody matter and is sometimes classified as what is called a "status offense" that justifies the government taking custody of a minor even though the minor has not committed a crime.