Do you really need to replace car seats after 5 years?

[Lozzer]

Just wondering. I have a car seat I bought for DS1 back in 2001. Both he and his brother used it and then it has been sitting in a cupboard for several years. Now DD1 is getting to 9 months and I was thinking to dust down the old seat and have DD1 use it.
I emailed Maxi Cosi about replacement car seat covers and they recommend replacing the whole car seat after five years. Something about plastic becoming brittle. Does anyone with a technical background know if this is true? The seat appears to be in good condition and it was never in a car that crashed.
Many thanks, Lorien