As culture has shifted over the last 100 years or so, we have been increasingly upping the age at which people are considered "adult". When I was in therapy school, we learned that the stages of development that were written even 20 years ago were pretty much invalid now. The two main stages that are developing are the time period post-retirement but prior to physical decline...the Baby Boomers have started redefining this from a blanket "retirement" to a time for second careers and such...and then the time after classic "young adulthood". This time period has been expanding because of later marriage/baby making. Whereas my mother got married 2 years after college graduation and had babies 2 years after that, I got married 6 years after graduation and had babies 3 years after that. Thus, my twenties were nearly entirely responsibility free and thus a distinctly different time of life than it was for my mother's generation. From what I can tell, my trajectory was not terribly deviant from the norm.
All that being said, it's getting harder to pinpoint exactly when someone stops being "young". I've noticed a tendency for people to push the age where youth is an excuse for less than advisable behavior higher and higher. I'm trying to suss out a consensus on this, so it's time for a poll! Or rather, polls! There's a few angles here: