I’m not as sure that it is as simple as others are portraying it.
First, moderate alcohol consumption is NOT intrinsically sinful.
Then there is fourth commandment, to honor thy father and mother. Does this mean to always do what your parents want you to, as long as it is not immoral? I would argue that, for adults, it does not. Legally, in the U.S., someone is an adult when they have obtained 18 years of age. That would mean that an 18 year old could, without sinning obey an order from his/her parents. So an 18 year old drinking against his/her parents will is not necessarilly sinful.
Then, there is the legal aspect. Obviously, it is illegal in the U.S. for teenagers to consume alcohol. If something simply being illegal were enough to be mortally sinful, then trivial things like not signaling a turn on a road with no traffic would be mortally sinful also, and I think most would agree that it is not. One might then argue that it is mortally sinful to violate some laws but not others. Then how can we judge which laws are more grave to violate than others? By the size of the penalty? If that were the case, then not signalling that turn would be more grave than underage drinking in some states. (For example, in my state, underage drinking=$180 fine; unsignaled turn=~$300 fine).
Don’t get me wrong, I’m not sure that it wouldn’t be a mortal sin for an underage person to drink, but I think, based on the argument that I presented above that it is not. That being said, Catholics are obligated to obey all just laws, and the government has the right to control substances such as alcohol, so, at a minimum, underage drinking would be a venial sin.