[SketchySeaBeast]

"I know plastics become more brittle with age but a modern car seat that's expired is going to be tons safer than a car seat from 2000"

How do you know this? Maybe there's technology (crumple areas) that only works when the plastic is relatively new or cushion areas that only are most effective before they degrade, thus making them less safe than good old fashion plastic. We see this with modern cars in their bumper technology - they take the first impact better, but they aren't as rugged as older models. Do you actually know the trade-off, or are you just assuming?

[dsfyu404ed]

There's a little bit of assumption about material design in there but it's obvious from the seat designs that they've come a long way. They're actually designed to fit the form of the body now (which is a big deal for safety). Old seats were little more than booster seats with integrated seat belts. It's like the difference between a 60s bucket seat and a modern racing bucket seat.