When you’re fifteen and sixteen and you get in trouble with the cops, your parents are the ones who are responsible for the consequences like paying fines. They may take away your allowance for awhile, but they are the ones who will need to sign papers, communicate with the court, and pay money. But once you turn eighteen, you are responsible for yourself. Your parents will not be required to pay any of your fines (or other consequences) on your behalf. They may choose to help you, and they likely will, but they are not obligated to do so.
You see, anyone seventeen or younger is considered a minor or a juvenile. They don’t go to jail but maybe juvenile hall. They may have to perform community service and pay a fine. Their records are expunged when they turn eighteen so it’s like they get a fresh start. But anyone eighteen and over is considered an adult. They are legally responsible for their own actions and if they are arrested, they will either need to post bail, or stay in jail.