Let the sarcasm begin!

Out of the mouths of babes…

After my son (4) got home today, he set his drink down in our computer room and then went into the living room to watch TV. Not 5 minutes later, he yells to us asking us to bring him his drink because his “legs hurt!” (Read: some cool Xiolin Showdown move is on and I don’t want to get up and get my own drink)
Our answer: “Get it yourself. Your legs aren’t broken.”
20 minutes later, when the show is done, he walks into the room, picks up his drink with a heavy sigh and says “Thank you!” while leaving the room. I raise my eyebrow at my husband, and he to me, as we both “Boy!” followed by a “Sorry!” from our son.

Now, I know who he learned sarcasm from, but my question is, Why could he wait until he was 6, like his sister did?