The thing is...I think people look down on current music (at least right now) because even if you try to avoid it, it's shoved down your throat by the internet, TV shows, commercials, movies, the radio, etc. So if older people are going out of their way to voice their opinions about the music it's mostly because media goes out of it's way to make them hear it in one way or another.
Exactly. It really comes down to two major developments over the last decade:
1. Delivery channels - Record companies now have (in addition to albums, radio, TV, and concerts) youtube, facebook, twitter and all other forms of digital delivery, not to mention itunes and other means to purchase music. Also, there has always been word of mouth, but there are way more mouths now because of the internet. Without it I would not have heard of Justin Beiber until very recently...and that's assuming he would have the same level of popularity despite a lack of internet exposure.
2. Advanced targeted marketing - there are far more ways to collect and dissect a consumer's tastes nowadays. This coupled with the delivery channels creates a flood into pop culture.
Today's pop stars are manufactured to be fed to consumers through the above. Sure, this has been true for decades, but the level of consumer exposure is largely unprecedented.
How did the Beatles get so big? Baby boomers. Their music appealed the the largest generation (read: largest segment of consumers) the marketplace had ever experienced. Right place...right time...right number of willing listeners.
This generation had children of their own, sometimes called echo boomers, creating a new massive consumer base. Why were the Ninja Turtles so big? There were a crap ton of kids to buy them (or parents to buy for them). These trends have bubbles as tastes of the consumers change. Remember when wrestling was huge in the late 90s? Echo boomers.
I'm not sure how large the millennial generation is compared to echo boomers, obviously the information is readily available, but whatever the size, it is sustainable enough for the heavily marketable likes of Justin Beiber. The rest of us just have to wade through the tide or look for other waters.
Better yet, get a jetski and enjoy life outside of concern for what other people like.
SIDE NOTE: I was going to share thoughts on how the internet gives voice to bands who would otherwise not be heard outside of a record company's promotion, but I've meandered far enough off track.