Debt collectors have gotten both bolder and sneakier as the years have gone by. They may attempt to sue you for a very old debt, but if the debt is old enough, they may not be able to do so legally. To read more about "Time-Barred Debts," see this page from the Federal Trade Commission:
Note that one of the key sentences on the page says,
"Although the collector may not sue you to collect the debt, you still
owe it." Some things will go away of you ignore them long enough.
Sadly, debt isn't one of those things.