Our brain grows and develops not so much as a function of time but of experiences. I don't think MRI images of modern day teens is really enough to base a conclusion on. It may be true that teens today show less development but the reason for that is not a foregone conclusion.

I think what the author is trying to say in his article is that teens are capable of much more than they are allowed to do. And that very fact may be what causes the results in the research you linked above.