And this is the great tragedy of cool.  Eventually a trend, once exlusive to those crazy enough to do it (ex. videogames, punk rock, Heelies), will catch the eye of the mainstream.  Once adopted by the masses, the trend loses its place as part of the group identity, and becomes "uncool."  

Think about it...how many kids will want Heelies once they become associated with "old people." (or at least, not the exclusive domain of kids)

This leads to new trends like "shock rock," where the trend tries to become so impalatable to the masses that it cannot be co-opted and mass-marketed.  

There was a great old Dragnet where they were interviewing some 12-year-old kid.  He couldn't place the age of the "bad guy," so when asked to guess, he said "I don't know, he was some really old guy, like, 40 or something..."