How long after a debt has been created can a creditor report to the credit bureau?

I had some medical bills from 4/10/01 that I didnt not pay in full. I was harassed for awhile by collections and then left alone. This debt was never reported on my credit record and has never showed up. Today on 2/14/2007 (6 years later) I receive a letter from a collections agency telling me [...] http://bit.ly/pkxT0b