Most car seats expire 6 yrs. after the date of manufacture. This expiration date is due to possible degradation of the plastic seat shell, harness and buckles, etc. over time. Older seats also do not have new safety and convenience features. Using a car seat past that point releases the manufacturer from liability should the car seat malfunction during a crash. *I* would not use an expired car seat.
This is just one reason why I don't care for most of the 3 in 1 car seats. They typically expire long before a child is able to ride without a booster. (Kids are safest in boosters until at least 4'9" and can pass the 5 Step Test. My DS finally hit 4'9" at 11, and my DD:10 has a long way to go.) Also, I don't care for most of them in the booster mode. The shoulder belt adjuster causes the shoulder belt to be a good inch or two away from the child's body, which isn't a good fit and doesn't provide adequate crash protection.
You're much better off getting a traditional convertible car seat, preferably one that harnesses past 40 lbs., and then buying a belt positioning booster down the road when the child outgrows the car seat.