Being the best but still not knowing anything

I'm going through a rough patch with our littlest one. I'm always the bad guy these days. With this one you have to stay on top of discipline constantly otherwise it will get out of hand.

He was apologizing to me after a round of calling me names. In his special way he said:

"You're the best. But you still don't know anything"

He is almost 6. Isn't that 10 years too soon