Adulthood. Where and How?

In class the other day, we talked about adulthood and how it seems to take longer to get there nowadays.  We read two articles which can be found HERE and HERE.  The first talks about how more people are waiting longer to get married, have kids, and even finish school.  The second is a chart showing a traditional breakdown of ages and their definitions, among other things.

In the first article, it says that some adults got married very early, and then told their kids that they should wait to get married because it is hard work and younger people aren't ready yet.  I was impressed that people had the patience to wait for the right person and circumstances for marriage.  Popular culture portrays a couple getting married as young as 21 and having a turbulent relationship afterwards.  It seems that realistically, many do have enough self-control to not get in a relationship that they will regret.

The second article shows the important issues that each age group faces.  However, how can one person define the conflicts that any other one person faces?  For many, the age ranges may not fit correctly.  There are definitely 18 year olds who do not know how to handle the pressures of school, despite being in it for so long.  In addition, there are people who are very old (80 years old and higher) who are still working and leaving a legacy for younger generations.  I think that some kids may be very advanced or pushed ahead psychosocially, and be interested in marrying at a young age, maybe as early as graduation out of high school.  How can these people fit a model?  Are they somehow unstable because they are ahead or behind the "normal" according to this chart?  Can this chart even be applicable at all?

The part that I didn't notice in either of these articles is an attempt to define adulthood.  In our class discussion, we decided that adulthood comes when a person takes on monetary and mental responsibility for their actions, choices, and lifestyle, as well as the ability to command some respect from others.  Responsibility and respect.  I can imagine that a middle-aged man would still seem childish if he didn't command respect and shoulder responsibility.  I can't imagine a middle aged woman acting that way, but I'm sure it could happen.  Can you have an adult who doesn't "act their age"?  Even if all the normal hoops are passed through, (marriage, kids, a job, buying a house, etc.) I think that a legal adult can still act childish and face the psychosocial conflicts that a 12-year-old might face.  In the same way, a person who is still very young, like Huckleberry Finn, can shoulder great responsibility and be more adult than the aforesaid example.  So, can a system be placed on these cases, or any case at all?