I think it'd be more accurate to say that people become less idealistic as they age. Whether Iron Guard, Green Peace, tie-dye, or Brown Shirt, a movement, which takes with it the greater part of or a sizable chunk of culture, attracts weak minds who barnacle themselves onto it as a means of producing movement in their lives, eliminating existential inertia.
In a way, I thought, this answer was very close to mine. I had said that people's lives and thinking becomes more constrained. The opposite of a constraint is a goal. When a goal has enough of a moral tinge to it (equality, heroism, racial diversity, racial purity, etc., etc.) it's called an ideal. One reason to ignore constraints is commitment to one of these "ideals." Conversely, one reason to accept more constraints is that one has given up some of one's commitments to these ideals. What we are seeing, perhaps, is a shift from one sort of moral principle to another. From moral goals (like brotherhood) to moral constraints (like the rights of others, which you would be violating if you tried to ram brotherhood down their throats). So becoming more constrained can very easily go with becoming in a sense less idealistic.
You find the same distinction, between moral goals and moral constraints, in this article. It's an interesting report on a paper by Scott J. Reynolds and Tara Ceranic in the November 2007 issue of the Journal of Applied Psychology. They found that people who characterize themselves as moral ("I'm a caring person") tend to indulge in cheating behavior in school or in the workplace more often than people who do not. How can this be? Are these people simply deluded? If you look at Reynolds and Ceranic's explanation of their results, you see that they are thinking of morality in terms of goals, not constraints. The psychology they see here is something like this: I'm a caring person. I want to help people. If I cheat on my exam, I will be more assured of becoming a doctor, and that will enable me to help people more. It's for the greater good. And besides, caring people like me are special...
There are these two radically different senses of what "a moral person" is, so different that one of these sorts of people can seem immoral to the other. One is the person person who pursues lofty moral goals, and might cut corners to get there. The other is the scrupulous one who never lies or cheats.
It occurs to me that there are also two different senses of being "an idealist." On the one hand is Greenpeace, the Red Guard, the Brownshirts. On the other hand... Well, admittedly, no one would ever call you an idealist just because you scrupulously refrain from ever lying to or cheating others. Idealism isn't just about The Right. It must also include some reference to The Good or The Best. But what if you try to refrain from ever defrauding or aggressively coercing others, and also believe that it truly is for The Best if people's rights are observed? It's true that these constraints prevent you from designing an ideal world and forcing it on them. But, you believe, people are fundamentally rational. And though they seldom care terribly much about the whole world, or pause to take in the big picture, their ties on the micro-level, their links to family and friends, are intense and profound. They will go to great lengths to make them happy. People can also be amazingly inventive and imaginative, if given a chance. In the long run, they will find what is best and do it. Most of the bad things in the world are caused by institutions that actually punish rationality, stifle innovation, and extinguish imagination, not because people are bad in themselves.
This too could be a sort of idealism. It is mine.