I was talking with a friend last night who had never heard that car seats can expire and just in case anyone else hadn't heard either I thought I would share the information. All car seats contain a manufacture date on them. Some of them will also contain their expiration date, if they do not the manual will or the general rule of six years can be used. After six years (or the date on your seat) the chair should be destroyed so that no one else tries to use it.
The reason for this is two fold, one, degradation of the materials, and two, safety factors. The materials used will degrade over time. There is a internet clip that shows a crash test of a car seat that is 10 years old, and the "child" is thrown out of the chair. The second reason of safety is just that over time the car seats are improved and the safety features enhanced.
I hope this can help someone out!