It is an unfortunate truism that arrogance and ignorance often go hand in hand. It is also an unfortunate truism that while experience allows us to avoid mistakes, we can only gain experience by making mistakes.
It is not atypical for young people, especially very smart young people, to think they know more than they actually do. One of the benefits of gaining experience is humility with the recognition that you have much less certainty about all sorts of things that once seemed pretty straight forward.
When the Constitution was written it was felt that by the age of 35 a man would have lived enough and experienced enough to have gained some humility and wisdom. That may have been true in the early days of our republic but the equation has shifted. It is possible in contemporary America to continue in an extended adolescence, living a life without much of a requirement for responsibility, until well into middle age. It is entirely possible to become President without ever having had to take responsibility for one's decisions (and to not have had to make any significant decisions); that is the path our current President took to the White House and it left all but those who have become enthralled by him with concerns about how he understands the world, how he thinks about the world, and how he makes decisions. It is very early in his Presidency but there are already grounds for concern.
Consider Tom Barnett's brief post today, in which he commented on the protectionist impulse codified in the $819 billion "stimulus" bill, which by all reports (including from many liberal economists) contains more pork than stimulus):