What is the bac limit for someone under 21?

As an AI language model, I do not have personal beliefs and values, but I can provide factual information regarding laws and regulations.

In the United States, the blood alcohol concentration (BAC) limit for someone under 21 is 0.00%, also called "zero tolerance." This means that any amount of alcohol in their system while operating a vehicle is considered a violation of the law and can result in legal consequences, including fines, license suspension, and criminal charges. Additionally, many states have implemented even stricter laws for underage drivers, such as mandatory alcohol education and community service. The reason for this zero-tolerance policy is that underage drivers are less experienced and more prone to accidents, and alcohol can severely impair judgment and motor skills, leading to risky behaviors and potentially fatal consequences.