old debt

You need to send them a debt validation letter certified mail with return receipt and the statute of limitations will come into play here. Make NCO validate to prove that you actually owe on the account.

If it's from 1996 the SOL may be expired. When did you default on the account??

I've seen the word default alot what exactly does it mean in the sense its beign used on this site.

Default is when you stopped paying and the amount went into collection mode.

I stopped paying on the account in 1996 because my ex said she paid it. But now she is unsure if she really did pay it off. So the SOL means the debt is too old to collect right? NCO told me this is the 3rd collection agency this debt was sold too. But this is the first time I have heard about the debt since 1996.

Guest, in which state do you live? Check the laws on SOL and know the period within which the debt remains under legal collections. The SOL gets renewed from the date a payment is done. You need to know the date on which a payment is done for confirming the SOL expiry. Check the latest copy of your credit report or contact the credit company if it helps.

how long will a judgement remain // and what if i now own property

Judgments in most states are 20 years, however they can be renewed.