Why do they call adolescence the age of stupidity?

In fact, it's some adults who say that. Adolescents are well aware that they are changing into adults so they behave as if they had the same independence as their parents. And sometimes they can go too far (like expecting to stay out late without saying where they're going) and since parents don't approve of this behaviour this can cause family rows. Although they're not much fun at the time, these rows can help young people to mature mentally.