Originally Posted by merlinextraligh

You don't need to know the mileage to know whether the existing chain and cassette need replaced.

Measure the chain for stretch, actually wear. If its "stretched" beyond 1/16th of an inch replace it. (See Sheldon Brown's or Parktool's website)

As for the cassette, replace it when it skips, or the shifting performance deteriorates, even when everything is clean and adjusted.

Thus I'd measure the chain. If it needs replaced, replace it. Once the new chain is on, the cassette will tell you whether it needs replaced.