You have two options:
1. Declare it
2. Don't declare it.

I am also under 21, and I have done both several times when coming back from overseas and have never had any alcohol confiscated. Even flying as a UM and having Agriculture inspect it, it was not taken. I have brought alcohol back at least 10 times in the last 7 years, and have never had any issues.

While I do believe that US Customs can confiscate alcohol, I have never seen it done.