That won't work for the bartender! I've seen bars and liquor stores closed down for selling to someone with a phony ID.
And again, "Mens Rea" does not apply to Liability Crimes. That is to say, the bar gets busted because Jr. got busted on a DUI or fatality. The bar's insurance company feels that the bar is too high a risk, and refuses to insure the bar. Thus, the bar will close. In other cases, I have seen bars get routinely busted for underage drinking, and the City refuses to issue them a liquor license. In the city I went to college in, the bar owner made the business decision that it was cheaper to pay the fines than to turn away underage drinkers. Stupid decision on his part.