John Taylor Gatto talks about this particular phenomena in his book "Weapons of Mass Instruction". Just looking at the way our school systems treats teens is enough to come to a full understanding of why teens develop so slowly. Most teens have no responsibility or any need for a real worry in their lives (beyond just living up to some societal expectations). Without actually being challenged as a complete human being (rather than just mentally in some small part of your brain), why would the brain need to develop?

I believe the most detrimental part is the fact that teens themselves often don't see themselves as "complete", "grown up", or as adults. This leads them to see and treat everything through a particular lens that they can excuse any behavior on. If from the start they were convinced they were adults (were complete and grown up), I believe they would take much more responsibility earlier on for their actions. I know I would've.