It's not that most teens are stupid; it's that most people are stupid, and teens have less of a filter.
That. Most people thinking that young teens are generally stupid always say they know some exception (or more likely, they are the exception - ever notice how many teenagers there are on the internet that think every other teenager is stupid?)
But yeah, it's just that most people are stupid, by the standards people are talking about in this thread. There are smart teens same way there are smart adults, and stpid teens and stupid adults. It's just that the average adult seems less stupid than the average young teen because they aren't as mature, the same way a smart adult seems smarter than a smart kid even though that smart adult was presumably a smart kid once too.