How old can a debt before collection is no longer possible?

Depends on the statute of limitations in your state. Even if the SOL has passed, they will still try to collect on it because they hope you're not aware of it and that you would be dumb enough to pay anyway.

If the SOL has passed, I would simply dispute the debt in writing and demand they cease and desist further communication as you will not pay it. This leaves them with a choice to sue you, but they wouldn't because you could assert a positive defense in that the SOL has expired.

Don't pay them anything or "ratify" the debt, because that could restart the statute, which then could set you up for a lawsuit.

Collections is always possible until it is paid.

The SoL just protects you from being sued. And the SoL doesn't restart after making a payment.

The SOL doesn't restart after a payment?

fyi i know u work for a collector and have alot of useful information.. so what exactly restarts the sol if anything? iv e read on some posts here wher the sol begins from the1st time u defaulted..so for ex: u default for awhile then go on a debt management program for awhile then can no longer afford teh pmts.. when would the sol start the very 1st default or thedefault from the debt management program?..just curious if any1 know for sure cant seem to find out exactly how that works anywhere on the net