Here's how I explain expiration dates. It seems to get a much better reception than the 'hype' that seems to imply that car seats immediately become ineffective the day they pass their magical expiration dates.
Car seat manufacturers cannot possibly be expected to track every seat they have ever made forever in perpetuity. If we start finding that a Maestro, for example, starts having a failure in crashes or in every day use after about two years of wear and tear, Evenflo will issue a recall and let everyone know that they need to repair or replace the defective part that did not last as long as expected. But if the same car seats start to see a chronic failure after seven years, Evenflo is not going to acknowledge or track the failure because those car seats are supposed to be out of service anyway.
The consumer can feel comfortable using the 6 year car seat for 5 years, 11 months, and 29 days, because the company and NHTSA continue to monitor the seat's performance and track reports of failure. So if a chronic issue is uncovered in that timeframe, the consumer will be informed and steps taken to correct it. But after 6 years and 1 day, the consumer no longer has that protection. So if a car seat has a 6 year expiration, and there's a piece of the car seat that tends to break or stop functioning or become flimsy after 6.5 years, there would be no way at all for the consumer to know about it.
If Graco is content with being responsible for their Turboboosters and Nautiluses for ten years, then I'm happy to use them that long. I am confident that if we start seeing failures in 9.5 year old seats, Graco will let us know.
So... this is my experience with expirations:
I had a Peg Perego infant seat with a 5 year expiration given to me shortly before my daughter was born. I knew the seat had very light use and was stored indoors between children. It was a "grandparent seat" for one family and was used about three times before I personally bought it for a friend to replace an expired Evenflo she'd been making do with. My friend then gave it back to me. It was due to pass its 5 year expiration right before my daughter was born. Knowing that most seats have no problem lasting for six years, I honestly didn't feel too bad about the idea of setting it on the garage shelf just in case I needed it in an emergency (like, I don't know, my car caught on fire with my car seat in it and I needed to use a different car to evacuate the area and needed a seat???). We inspected it SUPER closely to check for any signs of wear or degradation or compromised integrity of any kind. It was in perfect shape. So I put it in storage and forgot about it for a few months.
Shortly after its expiration (but still before six years) I got it out because I wanted something to help DD sit up for some photos and for some reason I didn't want to use her regular car seat. I couldn't detach the damn seat from the base to save my life. We ended up with four adults (two on the car seat and two on the base) yanking and pulling to separate the pieces. Once we got it off, we found that the plastic piece that the handle pulls on to release the car seat had lost some of its rigidity and it no longer pulled as far as it was supposed to. I have since found out that this tends to happen to the Peg Perego infant seats toward the end of their lifespans. No wonder Peg puts a 5 year instead of a 6 year expiration on them! They actually do "self destruct" after 5 years! I now have a healthier respect for car seat expirations. If another piece of the car seat which, say, held the harness in place or kept the buckle locked, had started to wear out, who's to say I would have known before it was too late and my kid was flying through the windshield? Or what if the release lever hadn't failed until the moment I needed it most in order to pull my child from a dangerous wreck? Perhaps due to being "near" failure and then pushed over the edge by the impact?