A Statute of Limitations defense is always waaaay more complicated than it seems on its face. It is no different in the medical billing context.
Most medical bills are derived from unwritten, implied contracts. For example, if you get into a car accident and go to the emergency room, the hospital will provide service before you execute a written contract binding you agree to pay a set amount for the service. This is an example of an implied contract. In Illinois, the statute of limitation for implied contracts is 5 years. See 735 ILCS 5/13-205. But be careful. The 5 years starts to run from the date of the last payment to the hospital for the services provided. Not the date of service. Also, if you moved out of the state for a period of time, the hospital may be able to defeat a statute of limitation defense. A statute of limitation defense can be problematic if the insurance company made payments to the hospital on behalf of the consumer. This issue is explored in Sexton v. Brach, 124 Ill.App.3d 202, 464 N.E.2d 284 (3d Dist. 1984).
The news is worse if the consumer signed a written agreement to pay the hospital bill. In this situation, the statute of limitation is 10 years. See 735 ILCS 5/13-206. However, the written agreement must be complete. If it lacks essential terms such as price, then a court might construe the written contract as an implied contract and the the statute of limitations would be only 5 years. See Schmidt v. Niedert, 45 Ill.App3d 9, 358 N.E.2d. 1305 (1st Dist. 1976).
In summary, the statute of limitations can be a complete defense to a medical bill. However, be sure to examine your fact pattern for the pitfalls described above.