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I'm sorry to hear about your situation. Please note that I can only answer your question. I cannot assist you with clearing this up, contacting creditors, etc..., as that's all "legal counsel", and representing individuals is against the terms of service of this site. That restriction is actually for your benefit, as there's not a conflict of interest.
First of all, you need to understand that a "charge off" is different than a debt forgiveness. A charge-off is an accounting term that they use to balance their books. If they actually forgive the debt (in which you will receive a 1099-C "cancellation of debt" form, then that would indicate that the debt was actually forgiven. Otherwise, even though it is charged off (which is an accounting process) that does not mean that it's forgiven you would need to find out whether or not this remaining balance was actually forgiven, rather than merely charged-off. As such, it is entirely legal for them to pursue a charged-off debt, and report said debt to the credit reporting bureaus. They can't pursue a forgiven debt.
As to whether or not you can fight it, you could certainly try in that if you could show that there were any contractual issues or contractual defenses that you would otherwise be able to raise, that's what you could bring up. The better thing would be to negotiate a lower pay off than the total amount, as it is generally more cost-effective for a debt collector to accept a settlement.
Now to be entirely honest, there's probably nothing that they could do to enforce this. As you're retired, your income is probably limited to exempt resources anyway (Social Security, IRAs, etc...) and they can't go after your home. It's a piece of paper, essentially. They can try to scare you into thinking that you have to pay "or else", but they can't send you to jail, can't garnish your retirement income. It's all bark and no bite. So while it IS legal for them to continue to pursue charged off debt (as charging something is an accounting term, not debt forgiveness) they still probably don't have any recourse against you (meaning they can't force you to pay, even with a judgment).
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!