Here is yet another theory about why people tend to become resistant to change as they age (go here for my first theory, and here for Jim’s). This one relies on the desire of rational people to conserve their limited mental space and time. If one has only a finite amount of mental resources to spend in thinking about stuff, it’s rational to allocate less time to those subjects on which new information or discussion is unlikely to generate change.
Consider, for example, my attitude about communism. I have been told once or twice that I am “closed-minded” about communism (and certain other subjects) because I’m resistant to having debates on the matter -- I reject the viability of communism out of hand. But when I was a freshman in college, I had many long discussions with communists and their leftist allies. I considered communism, and rejected it. I considered it again, and rejected it. I considered it a dozen more times, and rejected it. After a sufficient number of iterations, I began to doubt I would hear any argument that I hadn’t already found wanting. It’s difficult to imagine, at this point in my life, that any new debate on communism will bring me any great new insights. While I admit there’s some non-zero probability of a new argument or piece of evidence that changes my mind, the probability is small, and thus the expected return from my mental investment is tiny relative to the expected return from a debate on (say) intellectual property, a subject on which I remain ambivalent. I have only so much time and mental energy to expend thinking about such questions; why would I waste time on one where the effort expended is unlikely to make a difference? If that makes me closed-minded, then I contend that I am rationally closed-minded.
If other people are like me in this regard, then people will tend to become more set in their views as time passes. For most of us, there are diminishing returns to added investment in any given subject area. (An exception is the academic whose initial investments in thinking and learning about a particular subject allow him to create a “gravy train” of publications with extensions and applications of his ideas.) If we’re talking about politics and economics, the effect should be especially pronounced among people who (unlike me) find such topics incredibly boring. When the psychic benefits of thinking about something are small, the mind rationally closes on that subject after a smaller amount of mental effort, thereby leaving more time for thinking about sports, women, beer, and so on.