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When an 18 year old is arrested, their legal process is much different than someone who is 17 or younger. 18 is the age when a person becomes a legal adult, responsible for his or her own actions. Their parents or guardians are no longer legally required to be responsible for them, though we know they will be because that is what parents do. So if someone who is 18 or over is arrested, the parents are not obligated to help bail them out of jail. In fact, they do not even have to be notified of the arrest.
However, when someone 17 or younger is arrested, the parents are involved. The minor is released to their parents, but not by the means of bail. Bail is reserved for adults. The minor may have to do some community service or they may have fines to pay, but it is unlikely they will go to prison. Prison sentences for minors is for when they are being tried as an adult, and that is only the case for very serious crimes.