Originally Posted by
urbanknight
They expire 5-7 years from manufacture date due to the plastic breaking down and becoming brittle. I'm sure there is a safe margin, but we're lucky that my brother gave me his seat he got when he had his son 5 years ago. Our boy will grow out of it just months before its expiration date.
Ok, I could see the brittle aspect, especially since so many seats just stay in the car while it's parked outdoors. A lot of vehicle glass is UV resistant, but not everybody's, so it's a lot simpler for them to give an expiration date rather than say, "If you drive the cars on this list, you can keep it for 8 years; if you have one of these cars, you get 5 years; etc etc etc".