When do you become an adult?

As I have started college, I have been amazed by the behaviors, and things that I have seen take place on a college campus.  Things that would be more at home in a high school or often a junior high school, rather than at a college.

For example, my Economics class is considered a "basic" class.  For the most part, regardless of your major, you must take so many Economics classes, so most of the people in these classes are Freshmen and Sophomores.  Assuming that most of the ages of the majority of the people are between 17-20, would you think that you would find sticking gum to he underside of the desk amusing?

So my question is this, at what point do you become an adult?

Some would say that going off to college and being responsible for your own actions would be a good place to start, yet, the gum under the desks has me questioning otherwise.

As I sit in these classes and look around the room at the students who are approximately 10 - 12 years younger than I am and I wonder what state businesses and our country will be in, once they begin to move out into the world.  It also makes me wonder at what point parents thought that it was "wrong" to teach their children about responsibility and "acting" appropriately. 

We were allowed food and drinks in the classroom in my high school.  Sure, there were a few classes, because of the days activities that had days when you weren't allowed.  You could chew gum, drink a coke, and eat a Snickers, yet I'm pretty sure that I don't recall any gum stuck under the desks.  The bad thing is that most of the gum is "fresh" because I sit in the exact same seat everyday.  The first day of class - no gum.  Yet on Thursday, my 3rd day in that classroom, there were 3 pieces of gum stuck to the underside.

Apparently going off to college doesn't make people an adult.  Obviously it has little to do with age either.