When is old, old & when is old, new again?

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Something that has always intrigued me about dance/house music is the constant high turnover of music and the ongoing quest to source and play the newest tunes. I have always liked though going out, and hearing the odd old track thrown into a set. But what I wanted to know(especially from dj's) is when is it cool again to throw in an old track into a set? Does it depend on the track? Pick an old fave that was big, but didn't blow up on radio and get flogged to death. I would probably like to hear something from say 5 years ago, but not necessarily 2. Do any of you try and play an old obscure track that you stumbled across, just to see if anybody actually knows it and picks up on it? It just seems that other stlyes of music that have DJ sets, eg. Hip-Hop, Jazz-Dance, Lounge and RnB really relish in throwing in some oldskool vibes, yet it seems to me it is not done as much in house music, but I love it when it happens. Any Thoughts?