we're not your average f##king nonprofit™
One thing is State Statute of Limitations another thing is Credit Reporting Time Limit. This is usually why even when an old debt falls off a report (say in California where the law stipulates 4 years) it can still be collected upon by an agency. The key to understanding credit debt is to understanding the time limits attached to each one.
Another thing to consider is the state of contract, or where the bank is incorporated. The bank (a company) is chartered in a state, understanding that home states statute of limitations for debt/collections is also important. Follow the link above to the Federal trade commission and read up on a more concise explanation of what "Time-Barred" accounts are.