Responding to a summons is a formal process that varies by community, so you will want to seek advice from a local attorney or from the local law library. Typically, you are required to respond to each paragraph of the summons and then lay out your affirmative defenses to the suit as provided under the local rules of civil procedure. The fact that you call this debt a "charge off" is of no consequence in defending the suit. Hope this perspective helps!
You may be able to claim SOL - but I can't tell for sure from the facts you give. The statute of limitations to file a lawsuit to collect on a credit card debt is 6 years from the date of default; if the default (not the charge-off date, which is frequently - but not always - 6 months after the default) on the account was within the past 6 years, then they are within the statute of limitations. Here, if the debt was charged off in 12/2007, and the charge-off date is exactly 6 months after the date of default, that puts the default date in June, 2007. That means the SOL would run until 6/2013, and the case was filed just in time to beat the SOL.
So, check your financial records, if you still have them - when did you make your last payment? Did you charge anything on the card after the last payment? These are activities that will be used to determine the actual date of default, and therefore the date the SOL started to run. If you can prove that the default happened more than 6 years from the date the lawsuit was filed, then file a motion to dismiss with copies of your records proving the actual default date.
If you think the default happened at least 6 years before the lawsuit was filed, but can't prove it right now, file an Answer and raise the SOL defense (if you don't, you will lose the defense), then do discovery. Don't let the creditor rely solely on a post-default statement to prove its case. I'd get an attorney to help with this, though.
Best of luck.
I am only licensed in Indiana. The information provided is general information, and is not intended to be interpreted as individual legal advice. For advice specific to your circumstances, you should consult an attorney.