Sarcasm

Zachary was grumpy today. When I say grumpy what I really mean is that he was so negative and cranky that I was ready to really tie him, gag him and throw him in the closet. After many rude and disrespectful remarks and actions on his part he was sent to his room without dessert. He then proceeded to scream, "I want out. I want someone to read to me! I want a hug! Mommy, can I get out now? I want out!" Over and over again he screamed and cried like a two year old. He turned our whole house upside down.
Emily asked, "What's wrong with Zachary?"
I replied, "I think something crawled up his butt and died."
Emily: Like what?
Me: Probably a squirrel.
Emily just stared at me. Horrified. I had to explain that I was kidding. It was just a sarcastic expression.
Emily: So there isn't a dead squirrel up Zach's butt?

Sometime during this discussion Jason went into Zachary's room to read to him.
I walked in and Zach said, "Mommy, since I have been so good can I get out now?"
Seriously? Does he think screaming and disrupting everyone for two hours is being "so good". There was no chance I was going to let him come out. I ignored him but muttered on my way out the door, "Yeah, because you have been so good." I said it too myself and was clearly being VERY sarcastic.

A few minutes later, Zachary was playing in the living room. I looked at Jason, flabbergasted, "What is he doing out of his room?"
Jason: You told him he could come out because he was being so good.
Me: I WAS BEING SARCASTIC!
Jason (mind you he is smirking): I tried to explain it to him but he insisted that you let him out.

I really need to stop using sarcasm with my kids. At what age do they get it?