I have yet to see a single piece of independent research which proves that car seats need a 5 year expiry date, and remain convinced that it is a marketing ploy to get you to buy a new car seat for each new DC.
Ok, if a car seat was bought 10-12 years ago, then you may want to think about not using it, but if you are being offered a car seat that was bought in 2010, then you should just go for it.
What about the stage 1,2,3 car seats that are supposed to grow with your children? What is the point of those if the plastic starts to deteriorate after a couple of years?
You are not supposed to buy a second hand car seat from someone you do not know, as you dont know if it has been in a crash, but if you know for a fact that it has just had normal use, then it is fine.
Oh, and I am also quite careful about car seats. DS is in a massive Recaro extended rear facing seat after researching FF seats v RF seats. But I think that car seat expiry thing is a bit bollocks to be honest.