Example:
I work at a McDonalds. I get off work and go down the street to a local micro brewery to have a couple of drinks before I go home. I get carded by the bouncer when I go into the brewery. It also just so happens that the LLC who owns the McDonald's I work at also owns the micro brewery.
Another employee of McDonalds comes in. I assume she was carded by the bouncer, just like I was, and I offer to by her a drink but she never drinks it. A representative of the LLC finds out that she was in there (under age) and that I bought her a drink, and fires both of us from our jobs at McDonalds.
I think that is unfair. I can see firing the girl since she knew she wasn't 21. I can also see firing whoever was responsible to check ID's since they obviously did not check hers.
I do think it was unfair to fair the OP. He had no way of knowing she was underage nor did he have any duty whatsoever to inquire about her age or ID her. It wasn't illegal by any means but in my humble opinion, it was unfair.
Thank you for clarifying the situation.