The progression of popular notions in a culture happens, for the most part, in generation jumps. As far as prejudice goes, it's almost exactly as T-Rex says. This is why I would be surprised if gay marriage were legalized at the national level any sooner than the next ten or twenty years. By then, more baby boomers will have kicked the bucket, and more young people will have grown up to become voting age. And the thing about these young people is that they will have grown up living in a world where some states allow gay marriage, but not all of them. In all likelihood, they will be unmoved by scare tactics about how much society will crumble if we allow gay marriage, because they will have examples to look to.
There are also things like technology and political ideas that work this way. The technology you grow up with is just normal. The technology that gets introduced between the ages of 18 and 35 (or so) is really neat. After that, everything is just devil magic. If you are reading this blog at all, you probably are not old enough to see any technology as devil magic (except maybe cell phones, which can be devil magic at any age). But think about how your parents and grandparents react to technology. Maybe they double-clicks on internet links. Or they try to talk internet slang and completely fail, because it's basically a law of the universe that parents cannot talk the slang the kids are using these days. If you have tried to introduce your grandparents to the internet, it has probably been a disaster, and you answer questions all the time that seem like the stupidest thing in the world.
Of course, this isn't what happens all of the time. One of the "ascendingest" players in Kingdom of Loathing is a grandmother in her 50s. She has played through the game more than five hundred times. Though it is more difficult for people to learn new skill sets when they're older, it's not impossible, and people do it all the time.
Likewise, you're never too old to take up a progressive political cause like gay rights. Contrariwise, you're never too young to complain about kids these days, with their pants, and their music, then shake your cane at them as you yell, "You damn kids get off my lawn!"