I would say anywhere between the 50's and the 70's.
That is when music was born.
Jerry Lee Lewis
and tons more....
They were the pioneers of music as we know it. They came before mass production of crappy artist that will sell a ton of records now and never be heard of again.
Think about it many of the bands I named are long gone. But their music still goes on strong because of how great it is. Their music has transended though generations.
Now think of the music from the 80's - present that will be thought of as "new classics" in 20-30 years. I am not implying that it is all crap, I just don't see that many bands from the recent past that will be thought of as classics in twenty or thirty years.