I do remember thinking in a less rational fashion. I remember going various mental stages. I remember going through at least some of the ethical stages some philosophers use to model ethical development.

There is a difference between children and adults. There is a whole collection of events you go through to get to the stage where you are considered 'adult':

  • Learning to take decisions for yourself, without your parents taking ultimate responsibility.
  • Moving away from home, potentially for the first time experiencing real loneliness.
  • Going through a romance, learning what the love between man and woman (in the cliche variant, at least) is.
  • Learning to extend activities over time - not living only in the present, but focusing your attention on the future.
  • Making serious mistakes, and having to live with the consequences.
  • Learning to use abstract thought - the ability to work with abstracts, e.g. to play chess, is normally not fully developed until a child reach the age of 12.
  • In our society: Learn to read, and pick up the background necessary to be a normal member of society.
  • Spend the physical time necessary to get past the raging of hormones to being more guided by experience than feelings.
  • Taking a job, and thus leading a life where you are not directly dependent on a particular set of people.

All these experiences combine to form what we know as an 'adult'. Different people experience different amounts of each of them. For most people in the western societies, the point where they have enough experience (of various kinds) that the total weave becomes an 'adult' is somewhere in the range of 16 to 25 years of age. Age is coincidental, but the correlation of random and stereotypical events make it most often correspond.