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Question:

What is the blood alcohol concentration (BAC) at which underage drivers are legally considered to be driving under the influence?

A 0.02%
Explaination

In the state of Idaho, drivers who are under the age of 21 will be considered to be driving under the influence if their blood alcohol concentration (BAC) is at least 0.02%. (Note: 0.02% is considered the smallest percentage that is measurable with the usual breath tests.) [8-6 Driving Under the Influence, Chapter 8 How to Protect Your Driving Privileges, Idaho Driver's Manual]