Underage drinking is nothing new. Kids and teens under 21 have been experimenting with alcohol since forever. However, drinking illegally can have entirely different consequences when someone under the age of 21 is under suspicion or charged with drunk driving or driving under the influence.
When it comes to those underage and drunk driving laws, the same DWI/DUI laws do not apply. This is because a person under 21 is not allowed to have any alcohol in their system when behind the wheel, in comparison to a person of legal drinking age, they are allotted up to .08 BAC of the legal limit for driving a vehicle. This is what's known as a zero tolerance law.
The goal of these zero tolerance laws is to combat the real dangers of drinking and driving. Not all states implement per se laws that impact drivers found with .02 or lower BAC, the laws regulate that the police don't have to prove intoxication as long as the driver is above the stated limit. This would mean a breathalyzer would be sufficient evidence in proving underage DUI or DWI. For those accused of underage DUI, an early drunk driving conviction can have an effect on employment background checks as well as car insurance coverage.
Not all states have implemented zero tolerance laws in relation to underage DUI. However, the result is generally still the same. Those below the age of 21 are not allowed to have any level, no matter how low, of BAC in their system while behind the wheel. A quality criminal defense may be well-utilized if charged with underage DUI.
Source: dui.findlaw.com, "Underage DUI: Zero Tolerance Laws," Accessed March 26, 2018