Actually, I have seen prosecutors try cases (in Jefferson County), when the person blew under a .08. There is an either/or scenario in the statute: You have a BAC of .08 or higher; or, you do not have normal use of your faculties (paraphrasing but that's close). IMO, it's BS - why even have a .08 limit if you aren't going to abide by it either way. Most juries will recognize this and "walk" the person if they are under the legal limit with no allegations of other substances (e.g., pills). And you guys are correct, if you aren't of the legal age to drink, you are a minor and any detectable amount of alcohol in your system is an offense.
Here's another element that has never made sense to me: On a first offense, if you are under 21 and give a breath specimen, the DL suspension is for 60 days. If you are over 21 and give a breath specimen, it's over 90 days. I guess the motivation is that any amount is an offense if you are under 21, so encourage them to blow. However, it seems to me it ought to be more penal in you aren't even old enough to drink in the first place.