12 Year Old Scotch

A guy comes into a bar and asks the barman for twelve-year-old scotch. The barman thinks "This guy is pretty pretentious" and proceeds to pour him a drink of six-year-old scotch. He gives it to the customer who takes a drink, exclaiming, "This isn't twelve-year-old scotch, this is six-year-old scotch" The barman thinks, hey this guy knows what he's talking about, and the two of them get into a conversation about where the customer is from etc.. At one point an old guy, who was sitting at the other end of the bar comes over with a glass and hands it to the customer. The latter takes a drink, and spits it out. "This is piss!" he yells. The old guy nods and says, "Yeah, but how old am I?"