Legally, you are not allowed to bring alcohol in when you are under 21. Not to say that you can't try

I actually got busted doing precisely that 3-4 years ago when I was under 21 - customs selected me for a random inspection. Basically, all that happened is that I had to sign a paper saying that I was voluntarily turning over the alcohol to US Customs. Then, I had to follow them to the airport bathroom where they poured perfectly good gin down the toilet.

In other words, most likely you won't get caught. If you do, the consequences aren't too bad.