Yes, send them a letter requesting proof of such debt, otherwise it may be just a mistake. Many times the collection companies have no supporting documentation of the debt and just make these demands in hopes of getting something from the debtor. Usually, they can only collect on a debt that is 4 years old, or 4 years from the last time you made a payment to them. Good luck.
A collection agency can go back as far as the statute of limitations allows. In Texas, this would be 4 years from the last payment for a credit card debt. A debt that only came to the collection agency in 2000 is very likely too old to collect. If you don't think you owe it, and they won't prove it to you, don't pay them anything.