Why are 12-13 Olds Profiled As Idiots?

It seems that people are always labeling people between the ages 12 to 13 as obnoxious idiots. When you read any forum post anywhere people say things like this. "Oh this is something a 12 year old kid would do" Like why are people of that age given a bad name. Lots of people in that age group are very smart and mature young adults. Any Ideas?