What decades of music belong to the category of classic rock?

I was watching Glee this week and Rachel made a reference to One by U2 being a classic rock song. WHAT? That song is from the 90’s. When I was growing up classic rock was The Beatles, The Stones, music from the 50’s and 60’s and some 70’s.

Is the category classic rock perpetually redefined as time passes? Since classic rock in the 80’s was music from the 60’s, is music from 90’s now considered classic rock?

I tend to think that classic rock defines a specific time period and thus it is not changeable by time.