Many websites will claim a variant of the following claim:
Do not buy a used car seat unless you can verify the age of the seat. There should be a manufacturer's label on the back or bottom of the seat that gives the manufacture date and/or specific expiration date. All car seats and boosters have expiration dates.
Or something like:
No, it doesn’t suddenly become illegal to use a car seat at 10 years and 1 day after it was made, and there won’t be a warrant out for your arrest. But we know that you’d do anything to keep your sweet babe safe, and that’s why it’s recommended that you replace your car seat once it expires.
Is there any research supporting this statement? I.e. companies buying a bunch of used car seats from Craigslist, putting them into a car crash test and then seeing diminished performance. Or alternatively, independent companies buying brand new seats and comparing them in a car crash test to "expired" 10-year-old seats?
Update as requested: this question isn't asking "how do car manufacturers determine the expiration date" - this would be off-topic for Skeptics. My only focus is the claim that using an expired car seat is not safe.
"No longer safe to use" is ambiguous - does it mean 100% unsafe or 1% unsafe? But in common language it means that the threshold for the reduction in safety is low enough to recommend the product to be discontinued from use, without necessarily giving exact numbers. For the purposes of this question I'm interested in confirming whether or not expired car seats are more likely to cause injury/death than a new seat by a statistically significant margin.