This was in my basic psych class over ten years ago and when I went through the education program it was addressed in such theories as Demetriou and Piaget and many others. Piaget is the most famous and widely excepted. Physically most are fully mature by 17 and as young as 14 but mentally it can take as long as 22-24 to fully develop. Funny thing is that your cognitive ability declines pretty much as soon as it peaks just like physical development.It is now well accepted medically that the part of the brain that controls judgment does not fully develop until the early twenties.
So start quoting Piaget next time your teenager says their an adult. I have had the kids in school read it and while it is addressed in psych classes many really don't think about it. When you actually talk to them and have a debate they start to realize that maybe they could still learn a little form their parents and teachers.
What’s the difference between a graduating high school senior and a college freshman? A summer of drinking and three months.
When you look at it that way it is kind of hard to really look at an 18 year old as an adult but the law does and maybe that's not right but that's the law!
Edit: Then again look at our young service men and women between 17-19. We would defiantly call them adults but it doesn't change the fact that they are not fully mentally developed. It is a weird age your an adult but your not an adult.