"How Old Scotch?" joke

Angus McClod walks into a bar and asks for a bottle of forty-year old Scotch. The bartender, not wanting to go down to the basement and deplete his supply of the rare and expensive liquor, pours a shot of ten-year Scotch and figures that his customer won't be able to tell the difference. Angus downs the Scotch and says: "My good man, that Scotch is only ten years old. I specifically asked for forty-year old Scotch."
Amazed, the bartender reaches into a locked cabinet underneath the bar and pulls out a bottle of twenty-year old Scotch and pours the man a shot.
The customer drinks it down and says, "That was twenty-year old Scotch. I asked for forty-year old Scotch."
So the bartender goes into the back room and brings out a bottle of thirty-year old Scotch and pours the customer a drink. By now a small crowd has gathered around the man and is watching anxiously as he downs the latest drink.
Once again Angus states the true age of the Scotch and repeats his original request for forty-year old Scotch.
The bartender can hold off no longer and disappears into the cellar to get a bottle of prime forty-year old Scotch. Soon, the bartender returns with the bottle and pours a shot.
Angus downs the Scotch and says, "Now this is forty-year old Scotch!"
The crowd applauds his discriminating palate.
An old drunk who had been watching the proceedings with interest, raises a full shot glass of his own: "I bet you think you're real smart," slurs the drunk. "Here, take a swig of this."
Rising to the challenge, Angus takes the glass and downs the drink in one swallow. Immediately, he chokes and spits out the liquid on the barroom floor.
"My God!" Angus exclaims. "That tastes like piss!"
"Great guess," says the drunk. "Now, how old am I?"