I must say that I think there are multiple things happening at the same time ...
First off, age does play a factor but for very different reasons than it did for previous generations. Over the past 10 to 20 years music has been in a fairly static state where most new popular acts are just derivations of what came before them. If you're under 15 (or so) you probably would be oblivious to this, if you're between 15 and 25 you'd probably be somewhat aware of how stale things are, and if you're over 25 odds are pretty good that you're bored senselessly with everything that is being produced.
The second problem is that the music industry has become "American Idolized", meaning that the industry has stopped looking for established quality acts to sign and (mostly) focuses on finding young marketable groups to "develop". In the 80s and 90s many new rock groups were being signed in their late 20s after a decade of developing as musicians, while today most new groups are being aggressively marketed in their early 20s. While it may not seem all that important, a decade of discovering your sound and developing your skill will likely result in a much different product than 12 months of "development" from a studio.
Finally, part of being successful at on a wide scale is being generic enough that "everyone" likes you. While achieving popularity often means you're talented, and by many measures your product is probably high quality, it tends not to be particularly interesting for people who're looking for something unique or original. If you put it into terms of movies or videogames, very few of the top 100 highest grossing movies or videogames are particularly unique or original, and if you're looking for something a bit different you often have to look for it in less conventional places.
