Juvenile records are not automatically sealed upon your 18th birthday.
You must petition the juvenile court to have them sealed. You can do this by filing out a form and filing it with the juvenile court in the county in which you were convicted. Contact the juvenile court or an attorney in the county you were convicted, and ask them to send you a copy of the form used in that county.
Usually, the court will not charge a filling fee.
Check to see if they have any special filing requirements such as additional photocopies or the need to serve copies of the petition on any government agencies (such as the District Attorney's Office and the Probation Department), and get the correct information for filing by mail.