Junk debt buyers attempt to force settlement on "time barred" or "Expired Statute of Limitations" accounts. The actual date that the debt becomes delinquent is supposed to be reported to credit reporting agencies under FCRA/FACTA within 90 days of the delinquency.
WARNING! Debt reporting re-aging is the practice of reporting a bad debt account as more recent than it really is. Re-aging causes the FICO score to drop dramatically since the scoring model interprets the re-age as a more recent default.
Junk Debt Buyers press for payment of some amount to bring the debt back under the statute of limitations after which they can sue you and have a chance of winning.
It is important to note that State statute of limitations (SoL) for collections are different from FCRA/FACTA statutes of limitations for reporting the debt on your credit report.