It seems like each decade in the last century can be 'defined' by the style of clothes that people wore, or the music they listened to. Is it fair to say that culture changes that drastically between decades, and if so, has that rate of cultural change increased more than in previous centuries?
This is a question that's come up before in different ways, and it's difficult to come up with a satisfying answer.
Is it fair to say that culture changes that drastically between decades?
No. I'm not a fan of identifying things by their decade - it promotes the idea that there's a relatively static style of dress, music, etc. typical of the whole decade, with a big change at the end. What really tends to happen is that change occurs continuously, and we artificially draw a line between 1919 and 1920 (for example). There was no huge shake-up in fashion or music then, but many works of fiction and even non-fiction treat "1920" as a special date - it's a new decade, the beginning of the 1920s!
Sometimes trends do change right on the decade line. More often they change three or five or seven years into it, and then they colloquially get rounded down or up in order to make the idea of the decade work. When it comes to clothing or fashion plates between about 1800 and 1970, I can usually give a date that's accurate within a couple of years because these changes were always occurring.
has that rate of cultural change increased more than in previous centuries?
One thing we have to deal with is the fact that we are intensely biased toward seeing changes in the recent past, and against seeing them farther back. We're surrounded with images of the past hundred years (ish) and we have personal experience with earlier decades in our lives as well. Being attuned to the subtleties important to contemporary life, we spot them instantly.
I'm not prepared to commit to saying that styles in pop culture, music, clothing etc. have always changed at the rate we're familiar with from the 19th and 20th centuries. But the more that I study, the more I'm convinced that the rate of change has been higher than people think - the subtleties are simply more elusive to us because these factors are less familiar. To a lot of people, 18th century gowns are 18th century gowns - the clothes in this family portrait are indistinguishable from the clothes in this one, but by partway through my graduate program I was able to tell that they were from opposite ends of the century, and by now I can make even more nuanced determinations. It's all about familiarity.