People who only find one era of music enjoyable are usually short-sighted and too lazy to really hunt for good music.
It's easy to say old music is the best because all the shit has been weeded out. It's incredibly easy to find accessible, easy-to-listen music from the mid-20th since it's all in the past. It's a bit more difficult to find 'good' contemporary music without looking for it because nothing right now is being called "legendary" or "classic" or "memorable" across the board unless it's in an ironic manner. Praise for this generation's music will only be found like, 20 years from now. And at that point, the kids of that age will say only the music we have now is good, and what they have 20 years in the future isn't good in comparison.
Most of my friends who know jack shit about music constantly say how bad contemporary music is, or how hip-hop sucks now, or some other bullshit. Music has been the same forever, all of it's good, it's just what YOU like. What bothers me the most is kids who say classic rock is the best because they played Guitar Hero 3 and now they're the final word on classic rock, and they, for one, believe that Led Zeppelin is the best band, like, ever.
Like Del said:
Some just complain about the status of rap
They say it's average in fact
They wish the eighties was back
I say everything's everything
Nothin' stay the same
And yet, it is the same just
given a different name