To answer the poster's original question:
Every case like this is unique, so there is no good answer. Depends on the judge, the prosecutor, the local tradition. In my area, a judge will give you a greater amount of community service or a larger fine for a second offense. In other areas, they don't care and don't respect the zero tolerance laws and may rubber-stamp a $100 fine. So, you'll either have to wait and see or retain a local attorney who is familiar with the local procedure.
Since this thread has taken a somewhat philosophical turn, I feel a desire to chime in here.
Yes, it is illegal in all states for people under the age of 21 to consume and/or possess alcoholic beverages. Yet you can marry, smoke, vote, and join the military at the age of 18 when the law says you become an "adult". Why the inconsistency?
I remember a great con law case from law school. Used to be that states and localities had the ability to determine their own age limits when it comes to alcohol. See Craig v. Boren, wherein:
The interaction of two sections of an Oklahoma statute, Okla.Stat., Tit. 37, § 241 and 245 (1958 and Supp. 1976), [n1] [p192] prohibits the sale of "nonintoxicating" 3.2% beer to males under the age of 21 and to females under the age of 18. The question to be decided is whether such a gender-based differential constitutes a denial to males 18-20 years of age of the equal protection of the laws in violation of the Fourteenth Amendment.
Of course, such a gender-based differential was found to violate the Fourteenth Amendment. However, the question remains, why would Oklahoma have such laws in the first place? Well, I know the answer. Some say that it's because women are generally thought to "mature" quicker than their male counterparts. But the truth of the matter is that older men like younger women, so these laws allowed 21+ year old men to monopolize tavern access to 18 year old girls. None of those pesky 18 year old boys to compete against when going out on the town on a Friday night.
And these laws were not unique to Oklahoma; many "rural" states allowed young women to drink before young men. These laws were unconstitutional, thus illegal, thus we don't need to have any respect for them now. Just because the National Minimum Drinking Age Act of 1984 ("the Act") has been held constitutional and therefore legal, does not mean that we should automatically support or respect a nationwide 21+ limitation. In fact, many anti-federalists, libertarians, and republicans strongly oppose the Act based on their convictions, not their morals.
I, personally and morally, do not believe that it is "fair" that a man can die for his country (or, in the age of our occupation of Iraq, at least lose a leg or two) and not be able to lawfully consume a beer. I, professionally and ethically, question legislation which is passed only under duress that non-compliant states will lose their federal highway funding. I, academically and socially, support civil disobedience to the extent that each American citizen has the ability to choose whether or not to obey all laws, no matter how bizarre or seemingly unfair. It is more noble to question that which is imposed on us than to follow that which is directed to us.
And frankly, a kid drinking beer before 21 is either going to end up an alcoholic someday, or not. The age of first consumption will have very little, if anything, to do with later chemical dependency. It's more an issue of genes, social surroundings, and behavioral patterns.