I just don't believe that any changes that occur once a person hits their twenties are surmountable enough that there's any drastic difference in the way that that people in their late teens and early twenties behave. The 'character building experiences' that usually occur around that age are not actually exclusive to that particular age group and can occur much sooner or later depending on a wide variety of circumstances.
The kind of behaviour that most people would consider typical of teenagers is stil hugely prevalent amongsts university students who stayed in education their entire adult lives. Especially if they've been fortunate enough that they have never had to work. However, a large number of people in the UK enter full-time exmployment at 16 and quickly become a whole lot more mature than people who stayed in education, who are potentially much, much older.
Amazingly enough, those experiences are not actually a requirement for behaving like an adult. It's not unusual for somebody who is 18 to have more wisdom and good sense than somebody who is 21 despite not having as many life experiences due to internal factors, such as personality, temperament and interest as well as some external factors, such as a good education.
Human growth comes not from time, but from experience. And while people accumulate experience as they age, to pretend that people in their late teens are somehow radically different from people in their early twenties is bullshit, as people in that age range come from a huge variety of circumstances that will affect their growth as an individual. Some people will live on their own and be working two part-time jobs by the time they're 19, some people will still be living with their parents by the time they're 25. If you wanted to be truly fair, you would say that between the ages of 18 and 22 people tend go through a period of growth and personal development due to discovering independence and having to provide for themselves. However, this is not guaranteed to change a persons temperament, their views and beliefs and is far more likely to affect just the way that they view themselves and their self-worth. I think I can safely say that five years from now, my temperament, views and beliefs well remain largely the same as they are now. My lifestyle however will be very altered and I'll (hopefully) have a lot more self esteem and confidence. These things will come not from age, but from big changes in my life and the events that surround me. These factors will be external from me, they don't come from within. Those same external factors can and frequently do happen to people regardless of whether they're 18 or 28.
Sometimes no matter how much experience a person has, they might never learn a thing and as a result, never develope that much as a person. Because we're all so insanely different from person to person and come from such a huge variety of circumstances, it's insane to have such a dismissive attitude towards some people just because of their age. They may be teenagers, but they may well be far, far more intelligent, talented, well mannered and knowledgeable than we are.
Just because a person is younger does not mean they haven't had the life experience neccessary to behave like an adult. I know you guys have grown and changed a lot since you were 16, but I can name just as many people who have not.