Many would argue that with the rise in necessity of undergraduate/graduate degrees and the increasing median age of marriage adolescence has now extended well into your 20s (or that we need to recognize “emerging adulthood” as a period between adolescence and adulthood):
http://www.nytimes.com/2010/08/22/magazine/22Adulthood-t.html?_r=1&hp=&pagewanted=all
http://www.bbc.com/news/magazine-24173194