Getting back to the question at hand...
Not a jerk for letting management know they are screwing up.
Not a jerk for calling the cops once management didn't listen. They should have acted. How were they to know that Papasmurff wasn't a cop or some other law enforcement official? I may not have called the cops because I might want to go back to that establishment, but I see your reason for doing so.
I'm not going to get into the whole 21-18 drinking age thing because I have my own convictions on the subject. However, if they are under 18, they are categorized by the law as a minor.
Restrictions imposed on minors are typically justified by an assumption of diminished mental capacity.
That's not my definition, just quoting.