Q - What does the church say about underage drinking--not necessarily in regards to drunkenness?
A - Thanks for the question. I would like to point other readers to my answer to a previous question on whether getting drunk is a mortal sin. While you ask a question that takes that out of the picture, it is all too common a question and very relevant to most college students.
Now, as to your specific question. It depends (don't you love that answer).
Alcohol is not, in and of itself, an evil thing. So, having a beer is not, by itself, wrong. That being said, there are other circumstances that can make a neutral thing (alcohol) be used for an immoral purpose.
It is not a sin, if you are not breaking the law and drink moderately. Now, the Texas state law allows a minor to under the supervision of their parents or guardian. So, this would not be a sin, if you drink a beer or a glass of wine with your parents.
But, I am guessing you mean drinking as a minor without your parents or guardian around. If this is the case, then you are breaking the law. While some believe drinking-age laws are arbitrary, we must still follow them, because they are not unjust laws. Breaking the law is a sin, if it is a just law, therefore underage drinking without your parents around is a sin. What level of sin (mortal or venial) depends on other circumstances. Here are some examples.
Example #1 - Your know that your parents do not approve of underage drinking, but you drink anyway. Therefore you are also breaking commandment #4 - to honor your father and mother. By the way, even if your parents encouraged you to drink before you turn 21, you are still bound to follow the law.
Example #2 - Others you are drinking with are drinking too much or participate in unhealthy or immoral behaviors because of drinking. This is scandalous, because you are giving tacit approval to their actions (even if you don't approve or or participate in their actions) by your presence.
Example #3 - You have a history of not being able to control yourself around alcohol and thus you are putting yourself in a near occasion of mortal sin.
Example #4 - You drink and drive. Not only is this very stupid / sinful, but it is also breaking another TX law, because a minor cannot drive with ANY trace of alcohol in their system.
There are other examples, but this will do for our purposes here. Is underage drinking a sin? The simple answer is yes, it is (most likely) a venial sin when you break the law but do not drink to excess. But, other issues can increase the gravity of the sin to make it a mortal one.