When you talk about underage drinking and driving, there are two issues that coincide: underage drinking and driving while impaired.
ANYONE who is over the age of 10 should be charged with a DWI for driving with a BAC over .08, or when they are "impaired".
I kind of think that if you are of draft-able age, you should be held to the same standards, and receive the saem privileges, as an adult. I say "kind of think" because I could be persuaded that actual service should be the bright line.
Nevertheless, in most states, below a certain age, you can't drive below a certain age with any alcohol in your sytem.
The federal government requires states to adhere to some standards regarding impairment while driving, and witholds highway (?) funding if the states balk.
Which, I'll admit, raises the issue of "creeping federalism"...
As to teenage drinking being already illegal, Texas law distinguishes "Minor in Possession" from "DUI", and while both are "fine-only", both merit "detention", and IIRC, the various penalties that the magistrate can impose differ by statute.
I was fortunate enough to be taught by a father that alcohol should ideally be used in moderation, but never, in his own words, "to act a g****m fool"
I also beleive that any impairment while driving, whether because of drug or alcohol use, mental impairment, cell-phone use, CD changing, kid-spanking, or whatever, should have some kind of consequences.
Exactly WHAT those consequences should BE should be judged by the circumstances.
And, needless to say, how much you want the government to intrude...