The longer in life it takes people to arrive at their political leanings, the better their sense of judgment seems to be. How can anybody less than 25 possibly have seen enough to have an informed judgment about which side is right? More to the point, how can any young person think he has seen enough? The smartest conclusion a young person can come to about politics is that he doesn't have enough experience to really know what's what.
It's always been my impression that teenagers convinced of their own righteousness grow up to be adults who only see one side of a (political or any other kind of) argument.
Some kids never have a chance. Those with far left parents, so called red diaper babies, grow up seeing only one viewpoint and convinced of their own righteousness from the start. Babies born into conservative (and especially, religious) families are similarly exposed to only one viewpoint. The extent to which people from either group are able to later recognize the parochial nature of their own upbringings will demonstrate how open-minded they are.
(It is ironic that those most smug about being "open-minded" are in fact usually the opposite.)
Real open-mindedness is demonstrated by admitting that one's own side is wrong occasionally. By seriously considering opinions from other side. And by exposing oneself to other viewpoints. (Liberals tend to watch NBC, CBS, ABC, MSNBC, and read Paul Krugman; conservatives tend to watch Fox News and read George Will.)
The ultimate test of good judgment is whether someone is willing to change his mind based on new evidence. If such a person feels that facts which contradict his beliefs must be explained away with lame, convoluted reasoning, then he is closed-minded.
Another trait inversely correlated with closed-mindedness is how quickly someone will turn an argument personal. Sometimes, when people don't have logic on their side, they will call the other side a name (like "Nazis"). If you have good arguments, you needn't resort to name-calling.
A young person should make an effort to read both conservative and liberal opinions, and try to see the merits and drawbacks of each. He should get a sense of how the media spins things, and what they report and don't report. He should also get a sense of which types of people (i.e., smart of dumb) are on each side. Which side has more liars and corrupt politicians. Which side believes in shouting down opposing speakers on campuses and elsewhere.
A young person should also look abroad, and see which countries believe in a free press and free elections. And which countries rely on the military to keep their citizens in line.
Any 20 year old who feels he understand politics is severely deluded. The smartest thing he can do is have a strong sense of how little he knows.
I guess that last sentence pretty much applies to any 55 year old as well.