When you are 18 and you are caught drinking and driving , you will get charged way more than if a 21 or over person was caught. You will get charged more because you are underage so you should not have any alcholol in your system and also because you are driving under the influence.
When you are 18 you can sign legal contracts. You can open bank accounts and apply for loans on your own. If you break a contract, you could be sued and be required to appear before a judge, have an unpaid debt, and could file for bankruptcy.
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When you turn 18, it is your duty as a citizen to go to a jury trial.