The brain isn't fully developed until around age 25.* Various factors, such as trauma, parental styles, and having dumb friends can delay or stall brain development, so some brains aren't fully developed until later, or may never fully develop. While visiting for the holidays I told my parents that I now have an adult brain. They thought that I said that I have an addled brain.
In spite of their opinion I believe that my brain has improved significantly over the past two years. Reading over my blog I can definitely see this. In fact, I'm surprised that my adult friends have put up with me for this long. Even comments that I made a month ago on twitter make me do a facepalm.
One thing that seems strange is that we are sent to college before we can truly take advantage of the learning opportunities there.
*You can see the development in the young adult brain at MIT's site here: http://hrweb.mit.edu/worklife/youngadult/