Definitely an age thing, but is crazy making while they go through that stage. Our eldest son was the worst about it. We finally found catching him being honest and showing him that we weren't going to explode if he was just straight with us was what ultimately got him to turn around. That didn't mean there weren't consequences for his mistakes, but we made sure to discuss his errors calmly and to make those consequences logical and reasonable.

The best after effect was that when his younger brothers reached that stage, he told them how much easier life is if they just stay straight with us. They took his advice to heart, and while we haven't been completely lie free, it has been not nearly the problem we had with our eldest.