It's all about the people and how much free time they have. For example, I grew up in the '80s, with Madonna. As any teenage girl of the time, I loved her look and music. I liked only a bit of actual rock, and mostly pop. The '90s were eurodance for me, and 2000s were adult, popularized, alternative rock. Everything I listened until 2009 was under the monicker of "popular" and "mainstream".
In 2009 I started listening to underground indie bands, and today I mostly listen to artists that record music at home, and not only don't even have a contract with an indie label (let alone a major one), but they don't think they will ever sell a single digital unit, so they give their album for free on Bandcamp.
When all that took place in my head, within the last 2 years, I'm now INCAPABLE of listening to pop music. I hate it. I hate 95% of mainstream music. I feel that the bedroom artists, that don't have to answer to anyone, are the true heroes who PUSH the boundaries to explore new kinds of music.
I'm a 37 years old. I don't look like a hipster at all (more like a fat computer-stricken geek). But because I had the time and will (no kids you see), I took the time to educate myself about new kinds of music. In the beginning it felt like random notes, completely hookless, but as the time goes by and you get accustomed to the sound, a new musical world will show up in front of you. After that tipping point (it took me over a year to get sonically there), you can never go back to the old style of music. It will sound too little, too cheesy, too kitch, too unintelligent.