One thing I noticed when I was looking through my mom's 1973 high school yearbook was how similarly everybody was dressed.  Compare that to my (late '90s/early 2000s) high school experience, where there were different subgroups who dressed differently from one another. 

If you ever watch either of the MTV series 16 and Pregnant or Teen Mom, you'll notice how many of the dads dress similarly to one another: solid-color oversized baseball caps, usually turned to one side, and baggy clothes.  That's basically the style of lower-class white teenagers (you see those clothes all  over the men's section at Wal-Mart.)  On the other hand, my cousin, a conservative 17-year-old from an upper-middle-class family, would never be caught dead dressed like that.

In other words, what's changed is that in the past, people had a common culture, and teenagers dressed to distinguish themselves from 40-year-olds.  Now teenagers dress to distinguish themselves from other teenagers.