Actually - I am not sure we DO grow up in the same way we used to. I think we tend to remain far more flexible and "light" in our thinking and behaviour.
Strongly agree... Our rate of change seems to be increasing with each decade, so I don't think customs, traditions, or expectations are nearly as rigid. There are many more exceptions to almost all rules. It's a free-for-all!
Myself, I feel like I was grown up when I took charge ... at about 12 or so. I scrounged money, bought equipment, and went on a 3-week bicycle camping trip in another country when I was 13. The only other person involved was 15.
Public High School was about as crushing as prison, so I dove into an engineering career before graduating. Worked hard and saved up for a house on the beach at 23. After that, six years of college was a waste, jumping through meaningless hoops, so I never got a degree. Instead of "finishing my education", I started a never-ending process of curiosity and learning -- music, poetry, physics, economics, math, philosophy, history, psychology -- there's so much cool stuff out there!
Bankrupt at 26, retired millionaire at 33, lonely old woodsman at 36, are we really trying to create a schedule of expectations for people to follow? That's just weird.
The world is incredibly more complex today than two decades ago, so I would let the current generation take whatever time they need, to do whatever they need to do, in their own way. It'll sure be interesting!
Different milestones for different folks -- My favorite milestone is my first deliberate, conscious and deep relationship at 27. Love. Sweet closeness, beauty and joy, deep company. I'll never forget my first, but I was already grown up and looking for 15 years before the opportunity struck. That was luck, not me.
My skills constantly build, but my identity is still changing just as fast as twenty years ago, so I think maturity is for 21-year-olds trying to land a job. Whether 12, 27, later, ... the trick is to always be young. And to not feel like you're "supposed to" be anything, because truth is I don't think anybody fits in.