We are just going to do a quick and dirty (short explanation) on this topic. We get a few questions a month about this and we want everyone to always be safe with their precious cargo. Carseats do in fact have a shelf life and the simple reason for that is as follows.
That carseat is primarily composed of plastic and metal. It gets left in the car all year long, through various temperature changes. The plastic contracts and expands and over times is not as tight around the metal pieces as it originally was. This makes the carseat less effective over time.
Most carseats have their information all in the same location on the unit. Here is a sample from a Britax label. Check your unit thoroughly and make sure to abide by the date. Some insurance companies will not honour claims for damage or even injury if you are using an expired seat.