After spending last semester in spain I soooo wanted to bring back some wine. I was worried about this under 21 thing also. I took it onto the plane in Spain fine, then before getting off and going through customs in Chicago, I gave it to my friend who was 22. No problems. I was lucky that I was traveling with someone, so I didn't have to chance it. You could just put it inside your carry on before getting off the plane, and just not mention it, chances are they wont search your bag, if you are willing to take that chance.
Good Luck!
Kelly