Final answer:
The BAC threshold for underage drinker-drivers is 0.02% in most US states.
Step-by-step explanation:
The BAC threshold for underage drinker-drivers is 0.02 in most US states. This means that if an underage driver has a BAC of 0.02% or higher, they can be charged with a DUI (Driving Under the Influence). It is important for underage individuals to understand the risks and legal consequences of drinking and driving, and to always make responsible decisions.