Absent fraud (ie you never intended on paying the money back), threats of criminal action are just that, threats. You can't be arrested for failure to pay on a debt.
Additionally, if the company calling you is what is known as a "third party collector", that is someone who bought the loan from your original debtor, then they may not call you at work once you have informed them that your employer does not allow these type of calls. Additionally, you can also tell them not to call you at all, even at home. In fact you should tell them this by letter as well. Pursuant to the Fair Debt Collection Practices Act (FDCPA), if they continue to harass you by phone, you can bring a legal action against them. If the original debtor is calling you then the FDCPA does not apply, but after so long a time it is more than likely that it is a 3rd party collector that is contacting you. Even if it is the original creditor, there may be some specific state law protection for you on this.
Note: The FDCPA also prohibits a creditor from threatening criminal action.
As for the age of the debt, in the law there is something known as the "Statute of Limitations". What this does is bar a creditor from bringing lawsuit against you after a certain period of time has passed. In NE, for a claim of this kind, the S/L is 4 years. But the limitation's period can be started anew if a debtor acknowledges the debt. Here, if you tried to work on re-payment arrangements, I'm afraid that the statute may once again have been started. However, what act(s) actually re-starts the S/L varies from state-to-state.
At this point, I would speak to an attorney as to all of this. Even if you can still be sued on this loan, an attorney might be more effective in working out payment arrangements with them. They can also advise you what to do regarding any judgement that may be obtained.