Blood alcohol content

Blood Alcohol Content, often abbreviated BAC, is the percentage of blood that is alcohol. It is used as an objective (and ideally therefore, legally uncontestable) measure of the level of impairment of an invidiual, especially as it pertains to the operation of a motor vehicle.

In the United States, most (all?) states have a legislated BAC that defines inebriation at a specific percentage (often about 0.15 %). In some states, underage drivers are considered legally impaired at lower levels (perhaps 0.08%).

If you mean underage drivers as in people who aren't old enough to drive this seems odd - like shooting somebody and being charged for excessive noise. Do you mean that drivers below the drinking age have a lower limit (i.e. effectively no alcohol, because they're not supposed to be drinking?

see also: Alcoholic units