A new study reports there are cognitive benefits for children who learn to lie early in life. Researchers say children who learn to lie early have more self control, better executive function and improved theory of mind. ...This is a good example of psychologists doing too much mindreading. It is very unlikely that the 14-month old baby was trying to deceive his dad. Babies just don't have thought processes that complex. Maybe the baby can learn that a certain hand symbol has some relation to milk, but that's about all.
Children have been found to be capable of fibbing before they are seven years old, and some as early as two. He caught his own son lying for the first time at 14 months. He had taught him some American sign language, and their son gestured for more milk although he wasn’t hungry. “We ran over to the fridge to get milk and he went, ‘Ha ha ha!’ He started to laugh.”
Three-year-olds can lie. Probably all kids learn to lie by age four or so.