YANBU - expiry dates are twaddle - what's the expiry date on my driver's seat?
I'm pretty sceptical about a lot of things about child seats, but the idea that plastic shells which are structurally loaded fatigue is entirely sensible.
Your car seat is a metal frame which is subject to very little loading other than your own weight, and the consequences of it failing are pretty benign. In an accident you're mostly located by your seatbelt, which is secured to the structure of the car, not to the seat. In a very high energy impact from the rear it would be bad news if your seat collapsed backwards, but because the back of your seat is hinged by the back adjuster there's no serious fatigue risk there. Sideways you're located by your seatbelt, and in a frontal impact you're again located by your seat belt (in both cases, airbags also do some of the work).
In the case of a child seat, the seat is located by either isofix or the car's seatbelts, and the child is then located in the seat by a separate harness. The plastic structure which joins the child's harness to the mounting points is utterly vital to the child's safety: if that fails, the child is at immense risk. Plastics light enough to handle have a finite life, especially if exposed to heat and/or UV.
Competition seats are more structural for the driver, and unsurprisingly, the FIA standard for homologating competition seat has a five year limit:
- Seat life:
The usable life of an FIA homologated seat will be 5 years from the date of manufacture indicated on the seat label.
An extension of up to 2 further years may be authorised where the seat has been returned to the manufacturer for re-validation. Extensions will be indicated by an additional label affixed to the seat identifying the date on which the seat eligibility ceases, and validated by the manufacturer's quality inspection stamp.