It depends on the circumstances. The only thing you can do is to hire an attorney who will file pleadings with the court and move to modify the agreement or work with the other attorney to come to a mutual agreement.
The statute of limitations
does work in civil cases, but deals with the amount of time you have to bring a suit. The statute of limitations, however, mostly "runs" when dealing with criminal cases. There is a statute of limitations, usually 5 years, for bringing suit for debts.
If it is for child support, the "damage" was ongoing until a certain time, more than likely when the children became of legal age. If it was alimony
, the judgment is still there and running with interest and penalties.
Be aware, attorneys know the statute of limitations like no one else and will probably filed this in the nick of time to save any statute of limitation problem.
It will not go away, especially if it is child support. You should work to resolve it and get it off of your shoulders. It is up to you whether you hire an attorney or attempt to work out a deal to reduce the amount. I would suggest you be determined to put this behind you and get it taken care of.
Let me know if I can help any further.