It does seem that many people's taste in music freezes in their late twenties--and consequently so does the lens through which they view pop culture; for example, if the music of your youth was rebellion (e.g., rock music), then you have a harder time dealing with pure bubblegum pop and vice versa--this is why oldies radio stations exist.
Part of the reason for this is that keeping up with music is simply not a priority as you get older; you have more important concerns, like work and family. Plus, the independence that comes with age probably frees you to take on a much more diverse set of interests. Listening to music is one thing that most people can do, even if they're living in their parents' houses without a particularly large income.
On the other hand, it does seem that, at least among some classes of adulty people, people are trying to stay hip later into life, and still keep up on indie bands and that kind of shit. (Or maybe it just seems that way because I live in a city with a bunch of hipsters and trust-fund kids with money and no job.) I think part the reason for this is that we are more than ever a leisure class--all of our basic needs are fulfilled relatively easily; most of our consumption is directed at social cachet, which tends to fuel these sorts of things where exclusivity is born from knowledge.
Also, technology has made it very easy to acquire new music: you don't have to go to Tower Records anymore. You can hear new music with a few mouse clicks--it takes almost as little energy or effort as watching television, which people do in spades--and buy it with only slightly more.
But this alone isn't enough; people's interest in (new) music does still wane with age--just not as much as it used to. The problem is that while it's easier to find new music, it's still jsut as hard, if not harder, to find new music that is good--partly, because after you've heard a lot of music, it's harder for a thing to sound interesting or new. Also, most of the new music--for the same reason that it's easy to acquire: technology--is really bad, so you have dig through all that shit.
Actually, a lot of people can't really tell the difference, so it's probably more the first reason than the second reason.
