Why do carseats expire?
The plastic breaks down over time. It's no longer strong enough to handle the forces sustained in a crash.
When do carseats expire?
Most carseats expire 6 years from the date of manufacture (DOM). Some seats even have their expiration date stamped onto the seat itself. Read your owner's manual to find the lifespan of your specific seat.
What is the Date of Manufacture?
The date the seat was made. All carseats are labeled with the date they were made. It's usually on a white sticker on the side, back, or bottom of the carseat.
What should I do with an expired carseat?
If your area has a carseat recycling program, you can take it there. If not, destroy it. Take it apart as much as possible. Take off the cover and harness. Remove any screws. Smashing it with a sledgehammer is a wonderful idea as well :) You can even attack it with some power tools. When you're finished, put it all in a black trash bag and dispose of it.
I bought my carseat 4 years ago and it's already 6 years old. Can't I still use it?
No. Many people buy a carseat off a bargain website only to find it a year or 2 old already. Expirations are set in stone. It doesn't matter how long the seat was actually used. If you think you're getting a *great* deal on a carseat, check the DOM first. You might also want to check the DOM when you buy in a store. I've seen seats over a year old on store shelves, being sold at full price.
Here is a crash test video using a 10 year old Britax seat.