5 yr old manipulation

The new "rule" in our house is you have to eat your supper or no snacks the rest of the night. If you get hungry you have to eat your reheated supper. I realize this is probably already a rule in most of America's homes but it's new in ours. Last night Cade of course didn't eat his vegetable soup. So later in the evening he comes to me in the bedroom and says "Mom, I'm going to eat my vegetables but not the potatos" I assume he realized that food is a good thing when you're hungry and I said ok, tell your dad to get it. He said "I did, I just wanted you to know". So off Cade goes back to the kitchen. I'm thinking to myself that he really is growing up and that I can't believe how easy that was, a month ago a huge fight would have developed over veggies vs cookies. A minute later Scotty comes in the room and says "Remember the rule you just made about eating all your supper or no snacks? Cade has to eat his soup BEFORE he gets peaches." I proceeded to say "What are you talking about? He told me he was eating his vegetables and never even mentioned peaches". Apparantly, the manipulator realized we couldn't hear the other's conversations and he tried to use it to his advantage. He told his dad I said he could have peaches but he told me he was eating his veggies. He was counting on having the peaches gone before I came back to the kitchen and noone would be the wiser. Does this REALLY start at 5 years old?! I realize that peaches vs veggies isn't a life altering thing but he's only 5. Yes, he has started "stretching the truth" to fit his needs or blaming his sister/the dog/Spiderman for things he did so he won't get in trouble but adjusting his story to get things out of mom and dad? I thought we had at LEAST 4 or 5 more years before he figured that out. The story he told was thought out, planned ahead to get what he wanted out of the situation. My little boy is definately NOT a little boy anymore. Why didn't someone warn me that 5 was the age of mental development? I now have a whole new situation to pray about!!!