You appear to be under the impression that an old debt (6 years for credit cards in Indiana, generally) is somehow "invalid." This is not the case at all.
The expiration of 6 years from the first delinquency would be a defense if you brought it up after the collection company sues you in court. It does not bar the creditor from collecting the debt, and the debt does not "go away" or "become invalid."
If you don't plan on paying this debt, then you do nothing at this time. Nothing you could do right now would help your position of "not paying this." All you can hope for is that 6 years passes before this agency/firm files a lawsuit. If they're sending demands, I would bet that the 6 years is close but hasn't yet passed.
If you do plan to pay the debt, contact them and arrange payment. Tips for negotiation of settlement and payment are in the sticky post on this subforum.
