D.,
I asked at a local baby store about this and they referred me to this site: www.carseat.org
They briefly told me that a car seat should not be used longer than 10 years or after an accident and told me that those manufacturers that put in the 5-8 year range are taking into consideration temperature extremes of below zero to 100+. Since you live in FL also, according to this baby store in Ft. Lauderdale (and I asked my father-in-law who is an engineer), we would have a bit more shelf life since the temperature extremes down here are not as wide or frequent.
My understanding on on the legality is that the previous poster is right, you can't sue the car seat manufacturer if anything happens to your child while in it and it is expired. Good luck!
How old is too old for a safety seat?
There is some controversy about the "expiration" date for safety seats. All experts agree that a seat should be discarded and destroyed if it is more than 10 years old, even if it looks fine. Most manufacturers suggest replacing a seat 5 to 8 years after the date of manufacture, because current safety seats may have better safety features than older seats, such as a tether or air bag warnings. The date of manufacture may be found on a sticker on the seat (unless it has peeled off) and may be stamped into the plastic shell. However, don’t confuse patent dates, which can also be molded into the plastic, with the date the individual safety seat was manufactured. For more information, go to Expiration date.