A stand-up comic named Suzy Eddie Izzard used to have this bit about children and lying. She would say that everybody lies. “When we were kids we lied our heads off! [we’d say] ‘I didn’t do it! I was – I wasn’t – I was[n’t there]! I was on the moon! With Steve!’ And your dad’s going, ‘I haven’t even accused you of anything yet.’ ‘Oh, all right. Well – what – what the que- well – I – I – well – what?’ [he says] ‘Did you brush your teeth?’ ‘No – yes – whi – what’s correct? Anywa – yeah. I was [on the moon]!”1
Psychologists who study lying say that children as young as 2 and 3 years old lie. I mean, they’re not very good at it. They don’t really understand what you can see and prove for yourself – but they still lie to try to get what they want. By four years old, their brains develop a bit more and they have learned enough to have a bit of a sense of what the other person may or may not believe. By 7 or 8, their lies get downright plausible and they even start to prepare for the ever exacting follow-up questions.2
[1] http://lotusford49.tripod.com/markz/dressedtokill.html
[2] https://www.psychologytoday.com/us/blog/media-spotlight/201311/when-does-lying-begin