Laws will vary by state, but I am going to guess that anywhere in the US you would be considered a minor in posession and therefore breaking the law. As noted already you could declare it and hope the immigration officer does not notice/care that you are 18. You could also not declare it and hope customs does not go through your bag. The negative outcome of each would be they take away your wine. My further guess is that intentionally not declaring it could cause you more problems.

Just make sure it is really worth the risk of losing it, not to mention the hassle of having to haul it half way around the world.