You are lucky he just threw you out, dc laws are nuts, if a bartender serves someone underage, they are taken out of the place in handcuffs, get to spend a night in dc lockup (not fun) pay a few thousand in fines, and most likely get fired, because they jeopardized the bar's liquor liscense (nearly impossible to get a new one in dc) They are just covering their asses.
I have known a few bartenders who had to go thru this.