Someone bought my first beer for me when I was 20
Where were you at when this happened?
I was a freshman college student.
Since it is illegal to drink before the legal age of 21, you could have refused the drink. This would have been the responsible thing to do.
You don't know what it's like to be a freshman in college.
It's best to stand up for what you know to be right and avoid breaking the law. Drinking before the legal age can cause you and the other person unnecessary legal troubles.