At what age can kids truly understand the concepts of truth & lies?

The other day, I went into my 3 year old's room and it smelled horribly. I usually make a point of not asking questions that I already know the answer to, but without thinking, I said, "Did you poop?!?" I guess he could tell from the sound of my voice that I was not happy with him, and he said, "No" and I was shocked. He sometimes says untrue things joking around, or playing make believe, but this is the first time I ever remember him telling a blatant lie to me in an attempt to get out of trouble. We had a little talk about telling the truth and lying, and I'm not sure how much of it he understood. Then, at day care a few days later, he peed on himself, and when his teacher thought that he was wet but wasn't sure, she asked him if he was wet, and he said, "No". She then went closer to him, and saw that he definitely was wet, so that was a 2nd lie over similar cirstances within a few days. How would you handle this?