At what age do kids understand sarcasm?

The other day we did some yard work for our elderly neighbor. She asked for some of her shrubs to be cut to about a foot off the ground. Max was working on it, cutting them a bit too short. A friend of ours (engineer, detail guy) came up to him and showed him about where he should be cutting. Max looked at him and said "why don't you go in my house and get a marker and mark on each branch where exactly you want me to cut each one!" Oh, if only he was being sarcastic... that would have been awesome!