There's a little misinformation in this thread, through what I'm gonna say probably wont help the OP much.
Anyways, alcohol laws vary by state. There is no federal law that outright bans alcohol for those under 21. It's all done with state laws. I live in Texas, and in Texas if you're under 21, you're allowed to consume alcohol as long as one of you're parental guardians is with you and gives you permission. Yes, you can even drink a beer at the bar with your mom or dad if you're under 21, or drink wine at a family dinner at the olive garden.
Selling alcohol doesn't technically require you to have a TABC license here, but every employer that sells drinks will want you to have one. I believe you have to be 18 to get a TABC license.
But yeah, possession of alcohol if you're under 21 is pretty much against the law here. I'm not quite sure if the customs officers would take your drink though. I mean if you're just going from one plane to the next at the airport, and you had documentation to prove that, then maybe they wouldn't care. Of course they could just be jerks and take it. Cause you know if they take it then they get to drink it.